类与对象
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基本概念

class

每个类的定义都以关键字 class 开头，后面跟着类名，可以是任何非 PHP 保留字的名字。后面跟着一对花括号，里面包含有类成员和方法的定义。伪变量 $this 可以在当一个方法在对象内部调用时使用。$this 是一个到调用对象（通常是方法所属于的对象，但也可以是另一个对象，如果该方法是从第二个对象内静态调用的话）的引用。看下面例子：

Example #1 面向对象语言中的 $this 变量

<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this is defined (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this is not defined.\n";
        }
    }
}

class B
{
    function bar()
    {
        A::foo();
    }
}

$a = new A();
$a->foo();
A::foo();
$b = new B();
$b->bar();
B::bar();
?>

以上例程会输出：

$this is defined (a)
$this is not defined.
$this is defined (b)
$this is not defined.

Example #2 简单的类定义

<?php
class SimpleClass
{
    // 成员声明
    public $var = 'a default value';

    // 方法声明
    public function displayVar() {
        echo $this->var;
    }
}
?>

Example #3 类成员的默认值

<?php
class SimpleClass
{
    // 无效的类成员定义：
    public $var1 = 'hello '.'world';
    public $var2 = <<<EOD
hello world
EOD;
    public $var3 = 1+2;
    public $var4 = self::myStaticMethod();
    public $var5 = $myVar;

    // 正确的类成员定义：
    public $var6 = myConstant;
    public $var7 = self::classConstant;
    public $var8 = array(true, false);


}
?>

Note:

除此之外还有不少用于处理类与对象的函数，详情参见 类 / 对象函数

new

要创建一个对象的实例，必须创建一个新对象并将其赋给一个变量。当创建新对象时该对象总是被赋值，除非该对象定义了构造函数并且在出错时抛出了一个异常。

Example #4 创建一个实例e

<?php
$instance = new SimpleClass();
?>

当把一个对象已经创建的实例赋给一个新变量时，新变量会访问同一个实例，就和用该对象赋值一样。此行为和给函数传递入实例时一样。可以用克隆给一个已创建的对象建立一个新实例。

Example #5 对象赋值

<?php
$assigned   =  $instance;
$reference  =& $instance;

$instance->var = '$assigned will have this value';

$instance = null; // $instance and $reference become null

var_dump($instance);
var_dump($reference);
var_dump($assigned);
?>

以上例程会输出：

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

extends

一个类可以在声明中用 extends 关键字继承另一个类的方法和成员。不能扩展多个类，只能继承一个基类。

被继承的方法和成员可以通过用同样的名字重新声明被覆盖，除非父类定义方法时使用了 final 关键字。可以通过 parent:: 来访问被覆盖的方法或成员。

Example #6 简单的类继承

<?php
class ExtendClass extends SimpleClass
{
    // Redefine the parent method
    function displayVar()
    {
        echo "Extending class\n";
        parent::displayVar();
    }
}

$extended = new ExtendClass();
$extended->displayVar();
?>

以上例程会输出：

Extending class
a default value
类与对象
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PHP手册
PHP手册 - N: 基本概念

用户评论:

Manish Gupta (23-Oct-2011 06:07)

Some thing that may be obvious to the seasoned PHP programmer, but may surprise someone coming over from C++:

<?php
class Foo
{
$bar = 'Hi There';

public function Print(){
    echo $bar;
}
}
?>

Gives an error saying Print used undefined variable. One has to explicitly use (notice the use of <?php $this->bar ?>):

<?php
class Foo
{
$bar = 'Hi There';

public function Print(){
    echo this->$bar;
}
}
?>

 <?php echo $this->bar; ?> refers to the class member, while using $bar means using an uninitialized variable in the local context of the member function.

Marcus (05-Feb-2011 07:00)

Here's another simple example.

<?php
// PHP 5

// class definition
class Bear {
    // define properties
    public $name;
    public $weight;
    public $age;
    public $sex;
    public $colour;

    // constructor
    public function __construct() {
        $this->age = 0;
        $this->weight = 100;
    }

    // define methods
    public function eat($units) {
        echo $this->name." is eating ".$units." units of food... ";
        $this->weight += $units;
    }

    public function run() {
        echo $this->name." is running... ";
    }

    public function kill() {
        echo $this->name." is killing prey... ";
    }

    public function sleep() {
        echo $this->name." is sleeping... ";
    }
}

// extended class definition
class PolarBear extends Bear {

    // constructor
    public function __construct() {
        parent::__construct();
        $this->colour = "white";
        $this->weight = 600;
    }

    // define methods
    public function swim() {
        echo $this->name." is swimming... ";
    }
}

?>

Doug (07-Oct-2010 03:29)

What is the difference between  $this  and  self ?

Inside a class definition, $this refers to the current object, while  self  refers to the current class.

It is necessary to refer to a class element using  self ,
and refer to an object element using  $this .
Note also how an object variable must be preceded by a keyword in its definition.

The following example illustrates a few cases:

<?php
class Classy {

const       STAT = 'S' ; // no dollar sign for constants (they are always static)
static     $stat = 'Static' ;
public     $publ = 'Public' ;
private    $priv = 'Private' ;
protected  $prot = 'Protected' ;

function __construct( ){  }

public function showMe( ){
    print '<br> self::STAT: '  .  self::STAT ; // refer to a (static) constant like this
    print '<br> self::$stat: ' . self::$stat ; // static variable
    print '<br>$this->stat: '  . $this->stat ; // legal, but not what you might think: empty result
    print '<br>$this->publ: '  . $this->publ ; // refer to an object variable like this
    print '<br>' ;
}
}
$me = new Classy( ) ;
$me->showMe( ) ;

/* Produces this output:
self::STAT: S
self::$stat: Static
$this->stat:
$this->publ: Public
*/
?>

ben dot corne at gmail dot com (26-Apr-2010 08:35)

method calling context aware. By this I mean it will get treated differently while being in a new statement compared to being in a regular call.

Example:
<?php
class Foo {
  private $className = 'Bar';
 
  public function make() {
    return new $this->className();
  }
 
  public function callClassName() {
   $this->className();
  }

  public function className() {
    echo "foo\n";
  }

};

class Bar {
  public function hello() {
    echo "bar\n";
  }
};

$foo = new Foo();
$bar = $foo->make();

echo "expecting 'bar': ";
$bar->hello();

echo "expecting 'foo': ";
$foo->callClassName();
?>

even tough $this->className() is written two times in exactly the same way, the one contained in a new statement gets the className field and the other performs the actual method.

Anonymous (07-Nov-2009 02:54)

It is also simple to get or set a property with a name determined at runtime:

<?php
$e=new E();
$e->{"foo"} = 1; // using a runtime name
// is the same as doing:
// $e->foo = 1;
?>

moty66 at gmail dot com (16-Aug-2009 03:59)

I hope that this will help to understand how to work with static variables inside a class

<?php

class a {

    public static $foo = 'I am foo';
    public $bar = 'I am bar';
   
    public static function getFoo() { echo self::$foo;    }
    public static function setFoo() { self::$foo = 'I am a new foo'; }
    public function getBar() { echo $this->bar;    }           
}

$ob = new a();
a::getFoo();     // output: I am foo   
$ob->getFoo();    // output: I am foo
//a::getBar();     // fatal error: using $this not in object context
$ob->getBar();    // output: I am bar
                // If you keep $bar non static this will work
                // but if bar was static, then var_dump($this->bar) will output null

// unset($ob);
a::setFoo();    // The same effect as if you called $ob->setFoo(); because $foo is static
$ob = new a();     // This will have no effects on $foo
$ob->getFoo();    // output: I am a new foo

?>

Regards
Motaz Abuthiab

alex c (07-Aug-2009 08:17)

ok this really basic but I always forget this. I always get an error like:

Fatal error: Call to a member function on a non-object

when i deal with oops

if it were me finding the error i'd search the internet for hours and then it would occur to me, I'm putting my class operator inside a function, but i would define the class in global file.

so like this:
test.php
<?
include(class.php);
$class = new newclassname;

function function1(){
  $class->dofunc();
}
?>

you'll get some die errors and try and do this with function1,

function function1(){
  newclassname::dofunc();
}

but if you're using $this inside your class then you'll get another error on non object

so basically, all you need to do is:

function function1(){
  $class = new newclassname;
  $class->dofunc();
}

or

function function1(){
  global $class;
  $class->dofunc();
}

i know it's simple, but it always gets me!

the_french_cow at hotmail dot com (14-Jul-2009 02:54)

For those of us who are new to inheritance, private functions are not visible in an inherited class. Consider:

<?php
    class A {
        protected function func1() {
            echo("I'm func1 in A!<br/>");
        }

        private function func2() {
            echo("I'm func2 in A!<br/>");
        }
    }
   
    class B extends A {
        public function func3() {
            echo("I'm func3 in B!<br/>");
            $this->func1();
            $this->func2();  // Call to private function from extended class results in a fatal error
        }
    }
   
    $b = new B;
    $b->func3();  // Ends in a fatal error

// OR

    $b->func1();  // Call to protected function from outside world results in a fatal error
?>

If you want a function to be accessible in class B but not to the outside world, it must be declared as protected.

Notes on stdClass (09-Jul-2009 11:26)

stdClass is the default PHP object. stdClass has no properties, methods or parent. It does not support magic methods, and implements no interfaces.

When you cast a scalar or array as Object, you get an instance of stdClass. You can use stdClass whenever you need a generic object instance.
<?php
// ways of creating stdClass instances
$x = new stdClass;
$y = (object) null;        // same as above
$z = (object) 'a';         // creates property 'scalar' = 'a'
$a = (object) array('property1' => 1, 'property2' => 'b');
?>

stdClass is NOT a base class! PHP classes do not automatically inherit from any class. All classes are standalone, unless they explicitly extend another class. PHP differs from many object-oriented languages in this respect.
<?php
// CTest does not derive from stdClass
class CTest {
    public $property1;
}
$t = new CTest;
var_dump($t instanceof stdClass);            // false
var_dump(is_subclass_of($t, 'stdClass'));    // false
echo get_class($t) . "\n";                   // 'CTest'
echo get_parent_class($t) . "\n";            // false (no parent)
?>

You cannot define a class named 'stdClass' in your code. That name is already used by the system. You can define a class named 'Object'.

You could define a class that extends stdClass, but you would get no benefit, as stdClass does nothing.

(tested on PHP 5.2.8)

webmaster at oehoeboeroe dot nl (04-May-2009 01:03)

If you pass $this by reference and then assign a new value to it, it will not behave as you might expect as illustrated by this example:

<?php
class TestClass
{
    public $data;
   
    function setData(&$data) {
        $this->data =& $data;
    }

    function hasData(&$data) {
        $saved = $data;
        $data = true;
        $result = $this->data === true;
        $data = $saved;
        return $result;
    }
   
    function isDataOf(&$object) {
        return $object->hasData($this);
    }
}

$o1 = new TestClass;
$o2 = new TestClass;
$o1->setData($o2);
$o2->setData($o1);
var_dump($o1->hasData($o2));   // true as expected
var_dump($o2->hasData($o1));   // true as expected
var_dump($o1->isDataOf($o2));  // false even though $o1 is in fact the data of $o2
var_dump($o2->isDataOf($o1));  // false even though $o2 is in fact the data of $o1
?>

You can make this example work by replacing the hasData method with:

<?php
function hasData(&$data) {
    return $data === $this->data;
}
?>

However, although I've not tested this, I've been told that Zend Engine 1, i.e. PHP 4, will choke on the === parameter when comparing recursing objects.

ialsoagree (24-Apr-2009 04:03)

Unfortunately, Arpit's solution creates a new class and leaves the old class inaccessible. If you need access to members of the class you are in you'll be unable to get such access. This can be a huge problem.

However, there is a solution:

<?php
class MyClass {
   public $message = 'Hello';

   public function MyClassFunction() {
       function innerFunction(&$this_thing, $message = null) {
          $this_thing->message = (!is_null($message)) ? $message : $this_thing->message;
          $this_thing->echo_something();
       }
       innerFunction($this); // echoes 'Hello'
       innerFunction($this, '<br/>New Message'); // echoes '<br/>New Message'
   }
  
   public function echo_something() {
       echo $this->message;
   }
}

$class = new MyClass;
$class->MyClassFunction();
?>

By passing $this as a variable by reference, you can access members of the class and even update them. If you don't want to be able to update them, you can simply pass $this to the function but not as a reference.

Arpit (06-Feb-2009 02:14)

//try this code if you define a new class inside an object method than we can refer to "$class->message"
//unset this instance doesn't affected the previous one
//it will not report a fatal error

<?php

class MyClass {
   public $message = 'Hello';

   public function MyClassFunction() {
             function InnerFunction() {
          $class = new MyClass;
          print_r($class);
          echo $class->message;
          unset($class);//unset this doesn't affected the previous one or we can also use different name $classNew=new MyClass;
       }
       innerFunction();
   }
}

$class = new MyClass;
$class->MyClassFunction();
?>

ialsoagree (03-Feb-2009 12:23)

I think it's worth mentioning that if you define a function inside of an object method, that function cannot refer to "$this" - doing so will result in PHP reporting a fatal error:

Fatal error: Using $this when not in object context

<?php
class MyClass {
   public $message = 'Hello';

   public function MyClassFunction() {
       function InnerFunction() {
          echo $this->message; // Reports a fatal error
       }
       innerFunction();
   }
}

$class = new MyClass;
$class->MyClassFunction();
?>

This issue cannot be solved by using the Scope Resolution Operator if you're trying to access a variable:

<?php
class MyClass {
   public $message = 'Hello';

   public function MyClassFunction() {
       function InnerFunction() {
          echo MyClass::message; // Reports a fatal error
       }
       innerFunction();
   }
}

$class = new MyClass;
$class->MyClassFunction();
?>

Additionally, you can NOT create a public function to access that variable:

<?php
class MyClass {
   public $message = 'Hello';

   public function MyClassFunction() {
       function InnerFunction() {
          MyClass::echoSomething();
       }
       innerFunction();
   }

   public function echoSomething() {
      echo $this->message; // Reports a fatal error
   }
}

$class = new MyClass;
$class->MyClassFunction();
?>

Note that in this last case, the error is generated on the line below echoSomething function declaration, not at MyClass::echoSomething();

However, it is worth noting that when called directly, echoSomething works fine:

<?php
class MyClass {
   public $message = 'Hello';

   public function MyClassFunction() {
       function InnerFunction() {
          MyClass::echoSomething();
       }
       innerFunction();
   }

   public function echoSomething() {
      echo $this->message; // Echoes 'Hello'
   }
}

$class = new MyClass;
$class->echoSomething();
?>

hugo (@) apres (dot) net (17-Jan-2009 04:37)

A simple approach to Multiple Inheritance

You can give yourself something approaching multiple inheritance with the following class:

<?php

class inheritance{

  var $bases = array();

  static function error_die( $errno, $errstr, $errfile, $errline ) {
    $backtrace = debug_backtrace();
    $detail = $backtrace[4];
    var_dump( $backtrace );
    echo '<b>Fatal Error</b>: '.$errstr.' of class <b>'.$detail["class"].'</b> in <b>'.$detail["file"].'</b> on line <b>'.$detail["line"].'</b><br/>';
    die();
  }

  private function fatal( $text ) {
    set_error_handler( array( 'inheritance', 'error_die' ) );
    trigger_error( $text, E_USER_ERROR );
    restore_error_handler();
  }

  function __call( $name, $args ) {
    if( $this->bases )
      foreach( $this->bases as $base )
        if( method_exists( $base, $name ) )
          return $base->$name( $args );
    $this->fatal( "Call to undefined method <b>".$name."</b>" );
  }

  function __set( $name, $value ) {
    if( $this->bases )
      foreach( $this->bases as $base )
        if( property_exists( $base, $name ) ) {
          $base->$name = $value;
          return;
        }
  }
 
  function __get( $name ) {
    if( $this->bases )
      foreach( $this->bases as $base )
        if( property_exists( $base, $name ) )
          return $base->$name;
  }

  function __isset( $name ) {
    if( $this->bases )
      foreach( $this->bases as $base )
        if( property_exists( $base, $name ) )
          return isset( $base->$name );
  }

  function __unset( $name ) {
    if( $this->bases )
      foreach( $this->bases as $base )
        if( property_exists( $base, $name ) ) {
          unset( $base->$name );
          return;
        }
  }

  function inherits( $name, $args = '' ) {
    return array_unshift( $this->bases, new $name( $args ) );
  }

}

?>

Most of the qualities of multiple inheritance provided by this class are revealed by the following code:

<?php //test inheritance

class base0 {
  public $base0var;
  public $basevar;

  function base0declare() {
    echo 'I am base 0';
  }
  function basedeclare() {
    self::base0declare() {
  }
}

class base1 extends base0 { // simple linear inheritance here
  public $base1var;
  public $basevar;

  function based1declare() {
    echo 'I am base 1';
  }
  function basedeclare() {
    self::base1declare()
  }
}

class base2
  public $base2var;
  public $basevar;
  function based2declare() {
    echo 'I am base 2';
  }
  function basedeclare() {
    self::base2declare
  }
}
?>

Multiple inheritance is achieved by extending the inheritance class, and then in the __construct function placing calls to the "inherits" method of the inheritance class. Each call pushes an instance of the inherited class into an array var which functions as a LIFO stack. Using the magic methods, any failed method call, property set, get, isset or unset is intercepted by the inheritance base class which then attempts to resolve the reference. Object method name conflicts are resolved simply by the later inheritance masking the scope of the earlier inherited method. I recognize there are shortcomings to the approach I offer here, but it works for all my current multiple inheritance needs and offers simplicity and ease of understanding as benefits.

<?php
class base_test extends inheritance { // multiple inheritance

  function __construct() {
    $this->inherits( 'base1' );
    $this->inherits( 'base2' );
  }

} ?>

Here are some code fragments you can try out to test things.

<?php

$testobj = new base_test();
var_dump( $testobj );
$testobj->base2declare();
$testobj->base1declare();
$testobj->base0declare();
$testobj->basedeclare();
$testobj->base2var = 27;
echo $testobj->base2var;

?>

I'd be interested in hearing any comments.

chris (@) xeneco (dot) co (dot) uk (27-Nov-2008 11:25)

Regarding object inheritance:

I hope this helps someone, it should help if you're new to OOPS

<?php

class A {
   
    public $x = 'A';

    public function foo() {
        $b = new B;
        $b->bar();
        return $this->x;
    }
}

class B extends A {
    public function bar() {
        $this->x = 'B';
    }
}

$a = new A

echo $a->foo();    //A

?>

I was doing something similar to this (example is greatly simplified to show logic) and spent a long while trying to work out why I would always get 'A' and never get 'B'. Now, after a few weeks, I have revisited the problem and have worked out why:

The code 'new B' creates a new instance of class B. While class B extends class A, it is a new object and not an extension of the object created by 'new A'

The value of $x is set to 'B' within the object $b, but not in object $a.

If within A::foo(), one was to access $b->x then one would obtain the vale 'B', for example

<?php

class C {
   
    public $x = 'C';

    public function foo() {
        $c = new C;
        $c->bar();
        $this->x = $c->$x
        return $this->x;
    }
}

class D extends C {
    public function bar() {
        $this->x = 'D';
    }
}

$c = new C

echo $c->foo();    //D

?>

Jeffrey (08-Oct-2008 11:49)

A PHP Class can be used for several things, but at the most basic level, you'll use classes to "organize and deal with like-minded data". Here's what I mean by "organizing like-minded data". First, start with unorganized data.

<?php
$customer_name;
$item_name;
$item_price;
$customer_address;
$item_qty;
$item_total;
?>

Now to organize the data into PHP classes:

<?php
class Customer {
  $name;          // same as $customer_name
  $address;       // same as $customer_address
}

class Item {
  $name;          // same as $item_name
  $price;         // same as $item_price
  $qty;           // same as $item_qty
  $total;         // same as $item_total
}
?>

Now here's what I mean by "dealing" with the data. Note: The data is already organized, so that in itself makes writing new functions extremely easy.

<?php
class Customer {
  public $name, $address;                   // the data for this class...

  // function to deal with user-input / validation
  // function to build string for output
  // function to write -> database
  // function to  read <- database
  // etc, etc
}

class Item {
  public $name, $price, $qty, $total;        // the data for this class...

  // function to calculate total
  // function to format numbers
  // function to deal with user-input / validation
  // function to build string for output
  // function to write -> database
  // function to  read <- database
  // etc, etc
}
?>

Imagination that each function you write only calls the bits of data in that class. Some functions may access all the data, while other functions may only access one piece of data. If each function revolves around the data inside, then you have created a good class.

wbcarts at juno dot com (21-Aug-2008 02:11)

CLASSES and OBJECTS that represent the "Ideal World"

Wouldn't it be great to get the lawn mowed by saying $son->mowLawn()? Assuming the function mowLawn() is defined, and you have a son that doesn't throw errors, the lawn will be mowed.

In the following example; let objects of type Line3D measure their own length in 3-dimensional space. Why should I or PHP have to provide another method from outside this class to calculate length, when the class itself holds all the neccessary data and has the education to make the calculation for itself?

<?php

/*
 * Point3D.php
 *
 * Represents one locaton or position in 3-dimensional space
 * using an (x, y, z) coordinate system.
 */
class Point3D
{
    public $x;
    public $y;
    public $z;                  // the x coordinate of this Point.

    /*
     * use the x and y variables inherited from Point.php.
     */
    public function __construct($xCoord=0, $yCoord=0, $zCoord=0)
    {
        $this->x = $xCoord;
    $this->y = $yCoord;
        $this->z = $zCoord;
    }

    /*
     * the (String) representation of this Point as "Point3D(x, y, z)".
     */
    public function __toString()
    {
        return 'Point3D(x=' . $this->x . ', y=' . $this->y . ', z=' . $this->z . ')';
    }
}

/*
 * Line3D.php
 *
 * Represents one Line in 3-dimensional space using two Point3D objects.
 */
class Line3D
{
    $start;
    $end;

    public function __construct($xCoord1=0, $yCoord1=0, $zCoord1=0, $xCoord2=1, $yCoord2=1, $zCoord2=1)
    {
        $this->start = new Point3D($xCoord1, $yCoord1, $zCoord1);
        $this->end = new Point3D($xCoord2, $yCoord2, $zCoord2);
    }

    /*
     * calculate the length of this Line in 3-dimensional space.
     */
    public function getLength()
    {
        return sqrt(
            pow($this->start->x - $this->end->x, 2) +
            pow($this->start->y - $this->end->y, 2) +
            pow($this->start->z - $this->end->z, 2)
        );
    }

    /*
     * The (String) representation of this Line as "Line3D[start, end, length]".
     */
    public function __toString()
    {
        return 'Line3D[start=' . $this->start .
            ', end=' . $this->end .
            ', length=' . $this->getLength() . ']';
    }
}

/*
 * create and display objects of type Line3D.
 */
echo '<p>' . (new Line3D()) . "</p>\n";
echo '<p>' . (new Line3D(0, 0, 0, 100, 100, 0)) . "</p>\n";
echo '<p>' . (new Line3D(0, 0, 0, 100, 100, 100)) . "</p>\n";

?>

  <--  The results look like this  -->

Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=1, y=1, z=1), length=1.73205080757]

Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=100, y=100, z=0), length=141.421356237]

Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=100, y=100, z=100), length=173.205080757]

My absolute favorite thing about OOP is that "good" objects keep themselves in check. I mean really, it's the exact same thing in reality... like, if you hire a plumber to fix your kitchen sink, wouldn't you expect him to figure out the best plan of attack? Wouldn't he dislike the fact that you want to control the whole job? Wouldn't you expect him to not give you additional problems? And for god's sake, it is too much to ask that he cleans up before he leaves?

I say, design your classes well, so they can do their jobs uninterrupted... who like bad news? And, if your classes and objects are well defined, educated, and have all the necessary data to work on (like the examples above do), you won't have to micro-manage the whole program from outside of the class. In other words... create an object, and LET IT RIP!

ashraf dot samhouri at hotmail dot com (24-May-2008 02:35)

@info -- 20-April

This is because you requested class "b" before defining it, not because you defined class "b" before "a". It doesn't make a difference which class you define first.

info at youwanttoremovethisvakantiebaas dot nl (20-Apr-2008 11:40)

if you do this
<?php

$x = new b();

class b extends a {}

class a { }

?>
PHP will tell you "class b not found", because you've defined class b before a. However, the error tells you something different.... Got me a little confused :)

david dot schueler at tel-billig dot de (15-Feb-2008 02:16)

If you just want to create a new object that extends another object and you want to copy all variables from the father object, you may use this piece of code:
<?php
$father =& new father();
$father->a_var = "Hello World.";

$son = new son($event);

$son->say_hello();

class father {
    public $a_var;
}

class son extends father {
    public function __construct($father_class) {
        foreach ($father_class as $variable=>$value) {
            $this->$variable = $value;
        }
    }

    public function say_hello() {
        echo "Son says: ".$this->a_var;
    }
}
?>
This outputs:

Son says: Hello World.

So you dont have to clone the entire object to get the contents of the variables from the father object.

aaron at thatone dot com (16-Dec-2007 02:46)

I was confused at first about object assignment, because it's not quite the same as normal assignment or assignment by reference. But I think I've figured out what's going on.

First, think of variables in PHP as data slots. Each one is a name that points to a data slot that can hold a value that is one of the basic data types: a number, a string, a boolean, etc. When you create a reference, you are making a second name that points at the same data slot. When you assign one variable to another, you are copying the contents of one data slot to another data slot.

Now, the trick is that object instances are not like the basic data types. They cannot be held in the data slots directly. Instead, an object's "handle" goes in the data slot. This is an identifier that points at one particular instance of an obect. So, the object handle, although not directly visible to the programmer, is one of the basic datatypes.

What makes this tricky is that when you take a variable which holds an object handle, and you assign it to another variable, that other variable gets a copy of the same object handle. This means that both variables can change the state of the same object instance. But they are not references, so if one of the variables is assigned a new value, it does not affect the other variable.

<?php
// Assignment of an object
Class Object{
   public $foo="bar";
};

$objectVar = new Object();
$reference =& $objectVar;
$assignment = $objectVar

//
// $objectVar --->+---------+
//                |(handle1)----+
// $reference --->+---------+   |
//                              |
//                +---------+   |
// $assignment -->|(handle1)----+
//                +---------+   |
//                              |
//                              v
//                  Object(1):foo="bar"
//
?>

$assignment has a different data slot from $objectVar, but its data slot holds a handle to the same object. This makes it behave in some ways like a reference. If you use the variable $objectVar to change the state of the Object instance, those changes also show up under $assignment, because it is pointing at that same Object instance.

<?php
$objectVar->foo = "qux";
print_r( $objectVar );
print_r( $reference );
print_r( $assignment );

//
// $objectVar --->+---------+
//                |(handle1)----+
// $reference --->+---------+   |
//                              |
//                +---------+   |
// $assignment -->|(handle1)----+
//                +---------+   |
//                              |
//                              v
//                  Object(1):foo="qux"
//
?>

But it is not exactly the same as a reference. If you null out $objectVar, you replace the handle in its data slot with NULL. This means that $reference, which points at the same data slot, will also be NULL. But $assignment, which is a different data slot, will still hold its copy of the handle to the Object instance, so it will not be NULL.

<?php
$objectVar = null;
print_r($objectVar);
print_r($reference);
print_r($assignment);

//
// $objectVar --->+---------+
//                |  NULL   |
// $reference --->+---------+
//                          
//                +---------+
// $assignment -->|(handle1)----+
//                +---------+   |
//                              |
//                              v
//                  Object(1):foo="qux"
?>

alan at alan-ng dot net (10-Oct-2007 05:41)

The following odd behavior happens in php version 5.1.4 (and presumably some other versions) that does not happen in php version 5.2.1 (and possibly other versions > 5.1.4).

<?php

$_SESSION['instance']=...;

$instance=new SomeClass;

?>

The second line will not only create the $instance object successfully, it will also modify the value of $_SESSION['instance']!

The workaround I arrived at, after trial and error, was to avoid  using object names which match a $_SESSION array key.

This is not intended to be a bug report, since it was apparently fixed by version 5.2.1, so it's just a workaround suggestion.

Dan Dascalescu (27-Oct-2006 07:00)

If E_STRICT is enabled, the first example will generate the following error (and a few others akin to it):

Non-static method A::foo() should not be called statically on line 26

The example should have explicitly declared the methods foo() and bar() as static:

class A                                                                                                                            
{                                                                                                                                  
    static function foo()                                                                                                          
    {
...