(PHP 4 >= 4.2.0, PHP 5)
is_a — 如果对象属于该类或该类是此对象的父类则返回 TRUE
$object
, string $class_name
)
如果对象是该类或该类是此对象的父类则返回 TRUE，否则返回 FALSE。
Example #1 is_a() 例子
<?php
// define a class
class WidgetFactory
{
var $oink = 'moo';
}
// create a new object
$WF = new WidgetFactory();
if (is_a($WF, 'WidgetFactory')) {
echo "yes, \$WF is still a WidgetFactory\n";
}
?>
is_a() 函数已废弃，自 PHP 5 起请使用 instanceof 类型运算符。上例在 PHP 5 中会是这样：
Example #2 在 PHP 5 中使用 instanceof 运算符
<?php
if ($WF instanceof WidgetFactory) {
echo 'Yes, $WF is a WidgetFactory';
}
?>
webmaster at thedigitalorchard dot ca (14-Jan-2012 09:35)
What a farce, I must say. :)
PHP 5.3.9 adds an $allow_string parameter in order to handle strings being passed in, but controls the auto-loading behavior? Talk about confusing. Not sure if there's a committee discussing changes to core functions like this, but I feel that's a poor design. Don't get me wrong, I greatly appreciate the voluntary efforts of PHP developers, but this loosy-goosy approach to function prototypes is a recipe for disaster over the long term.
eitan at mosenkis dot net (12-Jan-2012 09:09)
As of PHP 5.3.9, is_a() seems to return false when passed a string for the first argument. Instead, use is_subclass_of() and, if necessary for your purposes, also check if the two arguments are equal, since is_subclass_of('foo', 'foo') will return false, while is_a('foo', 'foo') used to return true.
Aron Budinszky (07-Sep-2011 12:21)
Be careful! Starting in PHP 5.3.7 the behavior of is_a() has changed slightly: when calling is_a() with a first argument that is not an object, __autoload() is triggered!
In practice, this means that calling is_a('23', 'User'); will trigger __autoload() on "23". Previously, the above statement simply returned 'false'.
More info can be found here:
https://bugs.php.net/bug.php?id=55475
Whether this change is considered a bug and whether it will be reverted or kept in future versions is yet to be determined, but nevertheless it is how it is, for now...
p dot scheit at zweipol dot net (16-Jan-2007 09:44)
At least in PHP 5.1.6 this works as well with Interfaces.
<?php
interface test {
public function A();
}
class TestImplementor implements test {
public function A () {
print "A";
}
}
$testImpl = new TestImplementor();
var_dump(is_a($testImpl,'test'));
?>
will return true
martin dunisch (13-Feb-2006 08:02)
Workaround for older PHP-Versions:
function is_a($anObject, $aClass) {
return get_class($anObject) == strtolower($aClass)
or is_subclass_of($anObject, $aClass);
}
dead dot screamer at seznam dot cz (06-Feb-2006 07:44)
Why I test if class `A` inherit class `B` or implements interface `C` before I create class `A`?
<?
//That isn't work:
//1. function is_A()
if(is_A('A','B'))$a=new A;
if(is_A('A','C'))$a=new A;
//2. operator instanceOf
if(A instanceOf B)$a=new A;
if(A instanceOf C)$a=new A;
?>
zabmilenko at hotmail dot com (08-Oct-2005 01:18)
Lazy Instantiation using is_a() and php5
<?php
class ObjectA
{
public function print_line($text)
{
print $text . "\n";
}
}
class ObjectB
{
public function ObjectA()
{
static $objecta;
if (!is_a($objecta, 'ObjectA'))
{
$objecta = new ObjectA;
}
return $objecta;
}
}
$obj = new ObjectB;
$obj->ObjectA()->print_line('testing, 1 2 3');
?>
In the above example, ObjectA is not instantiated until needed by ObjectB. Then ObjectB can continually use it's creation as needed without reinstantiating it.
There are other ways, but I like this one :-)
cesoid at yahoo dot com (06-Oct-2005 03:01)
is_a returns TRUE for instances of children of the class.
For example:
class Animal
{}
class Dog extends Animal
{}
$test = new Dog();
In this example is_a($test, "Animal") would evaluate to TRUE as well as is_a($test, "Dog").
This seemed intuitive to me, but did not seem to be documented.