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else

经常需要在满足某个条件时执行一条语句，而在不满足该条件时执行其它语句，这正是 else 的功能。else 延伸了 if 语句，可以在 if 语句中的表达式的值为 FALSE 时执行语句。例如以下代码在 $a 大于 $b 时显示 a is bigger than b，反之则显示 a is NOT bigger than b：

<?php
if ($a > $b) {
    echo "a is bigger than b";
} else {
    echo "a is NOT bigger than b";
}
?>
else 语句仅在 if 以及 elseif（如果有的话）语句中的表达式的值为 FALSE 时执行（参见 elseif）。

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用户评论:

php at keith tyler dot com (05-Jul-2011 06:52)

This is valid syntax:

<?php
if ($a) print "a is true";
else print "a is false";
?>

A holdover from the bash-style compatibility in older PHP versions, perhaps.

smoldar at gmail dot com (28-Oct-2010 03:21)

You can use the if to make a Yes/No field, verify if the statement is real or not and show the correct option checked.

<?php if($variable == 'S') {?>
<input name="blah" type="radio" value="Y" checked="checked"> Yes
<input name="blah" type="radio" value="N"> No
<?php } else {?>
<input name="blah" type="radio" value="Y"> Yes
<input name="blah" type="radio" value="N" checked="checked"> No
<?php }?>

Larry H-C (17-Aug-2009 06:08)

When you escape out of HTML, you can get an UNEXPECTED T_ELSE error with the following:

Error:

<? if( $condition ) {
        dosomething;
   }
?>

<? else {
       dosomethingelse;
   }
?>

Correct:

<? if( $condition ) {
       dosomething;
?>

<? } else {
       dosomethingelse;
   }
?>

Apparently the compiler thinks a ?> <? breaks the connection between the } and the else

Theoden (25-Apr-2008 01:42)

At Caliban Darklock

I don't know if it is just improvements in the parser, but there is a negligible difference in the performance of "elseif" vs "else if" as of version 5. One thousandth of a second in your example and 8 thousandths if the eval statement is repeated 5 times.
If the constructs are in regular code, then there appears to be no difference. This leads me to believe that the difference in the eval code is from there being an extra parser token.

Also the main performance burden of recursive functions is the stack operations of changing the context. In this case I believe that it would parse to very similar (if not identical) jmp controls.

In summary, use your preference. Readability and maintainability rank far higher on the priority scale.

One Additional note, there appears to be a limit of the number of "else if" statements (perhaps nested statements in general) that php will handle before starting to get screwy. This limit is about 1100. "elseif" is not affected by this.

dormeydo at gmail dot com (12-Apr-2008 12:51)

An alternative and very useful syntax is the following one:

statement ? execute if true : execute if false

Ths is very usefull for dynamic outout inside strings, for example:

print('$a is ' . ($a > $b ? 'bigger than' : ($a == $b ? 'equal to' : 'smaler than' )) .  '  $b');

This will print "$a is smaler than $b" is $b is bigger than $a, "$a is bigger than $b" if $a si bigger and "$a is equal to $b" if they are same.

mitch at mitchellbeaumont dot com (24-Jul-2007 08:09)

At gwmpro at yahoo dot com

The curly brace is not required however, for readability and maintenance, many developers would consider it bad style not to include them.

Caliban Darklock (08-Nov-2004 07:24)

If you're coming from another language that does not have the "elseif" construct (e.g. C++), it's important to recognise that "else if" is a nested language construct and "elseif" is a linear language construct; they may be compared in performance to a recursive loop as opposed to an iterative loop.

<?php
$limit=1000;
for($idx=0;$idx<$limit;$idx++) 
{ $list[]="if(false) echo \"$idx;\n\"; else"; }
$list[]=" echo \"$idx\n\";";
$space=implode(" ",$list);| // if ... else if ... else
$nospace=implode("",$list); // if ... elseif ... else
$start=array_sum(explode(" ",microtime()));
eval($space);
$end=array_sum(explode(" ",microtime()));
echo $end-$start . " seconds\n";
$start=array_sum(explode(" ",microtime()));
eval($nospace);
$end=array_sum(explode(" ",microtime()));
echo $end-$start . " seconds\n";
?>

This test should show that "elseif" executes in roughly two-thirds the time of "else if". (Increasing $limit will also eventually cause a parser stack overflow error, but the level where this happens is ridiculous in real world terms. Nobody normally nests if() blocks to more than a thousand levels unless they're trying to break things, which is a whole different problem.)

There is still a need for "else if", as you may have additional code to be executed unconditionally at some rung of the ladder; an "else if" construction allows this unconditional code to be elegantly inserted before or after the entire rest of the process. Consider the following elseif() ladder:

<?php
if($a) { conditional1(); }
elseif($b) { conditional2(); }
elseif($c) { conditional3(); }
elseif($d) { conditional4(); }
elseif($e) { conditional5(); }
elseif($f) { conditional6(); }
elseif($g) { conditional7(); }
elseif($h) { conditional8(); }
else { conditional9(); }
?>

To insert unconditional preprocessing code for $e onward, one need only split the "elseif":

<?php
if($a) { conditional1(); }
elseif($b) { conditional2(); }
elseif($c) { conditional3(); }
elseif($d) { conditional4(); }
else {
....unconditional();
....if($e) { conditional5(); }
....elseif($f) { conditional6(); }
....elseif($g) { conditional7(); }
....elseif($h) { conditional8(); }
....else { conditional9(); }
}
?>

The alternative is to duplicate the unconditional code throughout the construct.