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(PHP 4, PHP 5)

if 结构是很多语言包括 PHP 在内最重要的特性之一，它允许按照条件执行代码片段。PHP 的 if 结构和 C 语言相似：

<?php
if (expr)
  statement
?>

如同在表达式一章中定义的，expr 按照布尔求值。如果 expr 的值为 TRUE ，PHP 将执行 statement，如果值为 FALSE ——将忽略 statement。有关哪些值被视为 FALSE 的更多信息参见转换为布尔值一节。

如果 $a 大于 $b ，则以下例子将显示 a is bigger than b：

  <?php
 if ( $a  >  $b )
  echo  "a is bigger than b" ;
 ?>

经常需要按照条件执行不止一条语句，当然并不需要给每条语句都加上一个 if 子句。可以将这些语句放入语句组中。例如，如果 $a 大于 $b ，以下代码将显示 a is bigger than b 并且将 $a 的值赋给 $b ：

  <?php
 if ( $a  >  $b ) {
  echo  "a is bigger than b" ;
   $b  =  $a ;
}
 ?>

if 语句可以无限层地嵌套在其它 if 语句中，这给程序的不同部分的条件执行提供了充分的弹性。

用户评论:

[#1] brian at webdesignacademy.co.za [2015-06-04 10:25:57]

You can also check alphabet characters like this

<?php // Alphabetical Comparison $a="brian"; $b="zebra"; if ($a < $b){ echo $a." is before ".$b." in the alphabet"; } else{ echo $a." is after ".$b." in the alphabet"; } // Result : brian is before zebra in the alphabet ?>

[#2] pouyapanahzadeh at gmail dot com [2015-05-06 06:53:53]

[#3] synnus at gmail dot com [2015-04-21 09:54:54]

you want cr??ate IF in for with multi var :

(#) ? A : B; or ((#) ? A : B);
but is possibli this
((#) ? A.B.C.D : E.F.G.H);
exemple

(( $A>=$B ) ? ($A++) . ($B--) : ($A--) . ($B++) );
echo $A, PHP_EOL;
echo $B, PHP_EOL;

exemple :
<?php For( $A=0 , $B=10 , $C=50 ; $A<=$C ; $A++, (( $A>=$B ) ? ($A++) . ($B--) : ($A--) . ($B++) ) ) { echo $A, PHP_EOL; echo $B, PHP_EOL; echo $C, PHP_EOL; } ?>

[#4] Aldee Mativo [2014-02-21 03:52:20]

If you intend only a single line (one-way) conditional script, you can do the following:

empty($user) || $user->save();

if $user variable is empty, it will not execute $user->save(). Otherwise it will perform saving operation. In other words/form of expression, it can be expressed like this:

if(!empty($user))
$user->save();

[#5] robk [2013-05-20 01:02:05]

easy way to execute conditional html / javascript / css / other language code with php if else:

<?php if (condition): ?>

html code to run if condition is true

<?php else: ?>

html code to run if condition is false

<?php endif ?>

[#6] ehsan at chavoshi dot com [2012-04-09 16:05:35]

You can use a simple if and echo structure :

$i==1 and print "i is 1"
is identical with
if ($i ==1)
echo "i is 1";

[#7] sofwan at sofwan dot net [2012-02-28 09:31:33]

It seems that only numbers can be compared between them but actually an alphabet can be compare too. For example :

<?php // Number comparison $a="C"; $b="X"; if ($a<$b) { echo $a."is smaller than".$b; } // Result : C is smaller than X ?>

[#8] Donny Nyamweya [2011-02-11 20:30:25]

In addition to the traditional syntax for if (condition) action;
I am fond of the ternary operator that does the same thing, but with fewer words and code to type:

(condition ? action_if_true: action_if_false;)

example

(x > y? 'Passed the test' : 'Failed the test')

[#9] Christian L. [2011-01-25 10:58:14]

An other way for controls is the ternary operator (see Comparison Operators) that can be used as follows:

<?php $v = 1; $r = (1 == $v) ? 'Yes' : 'No'; // $r is set to 'Yes' $r = (3 == $v) ? 'Yes' : 'No'; // $r is set to 'No' echo (1 == $v) ? 'Yes' : 'No'; // 'Yes' will be printed // and since PHP 5.3 $v = 'My Value'; $r = ($v) ?: 'No Value'; // $r is set to 'My Value' because $v is evaluated to TRUE $v = ''; echo ($v) ?: 'No Value'; // 'No Value' will be printed because $v is evaluated to FALSE ?>

Parentheses can be left out in all examples above.

[#10] techguy14 at gmail dot com [2011-01-06 13:39:11]

You can have 'nested' if statements withing a single if statement, using additional parenthesis.
For example, instead of having:

<?php if( $a == 1 || $a == 2 ) { if( $b == 3 || $b == 4 ) { if( $c == 5 || $ d == 6 ) { //Do something here. } } } ?>

You could just simply do this:

<?php if( ($a==1 || $a==2) && ($b==3 || $b==4) && ($c==5 || $c==6) ) { //do that something here. } ?>

Hope this helps!

[#11] admin at leonard !spam challis dot com [2010-11-22 04:41:01]

When using if statements without the curly braces, remember than only one statement will be executed as part of that condition. If you want to place multiple statements you must use curly braces, and not just put them on the same line.

<?php if (1==0) echo "Test 1."; echo "Test 2"; ?>

Whereas some people would expect nothing to be displayed, this piece of code will show: "Test 2".

[#12] Rudi [2010-09-14 13:14:09]

Note that safe type checking (using === and !== instead of == and !=) is in general somewhat faster. When you're using non-safe type checking and a conversion is really needed for checking, safe type checking is considerably faster.

===================================
Test (100,000,000 runs):
<?php $start = microtime(true); for($i = 0; $i < 100000000; $i++) if(5 == 10) {} $end = microtime(true); echo "1: ".($end - $start)."<br />\n"; unset($start, $end); $start = microtime(true); for($i = 0; $i < 100000000; $i++) if('foobar' == 10) {} $end = microtime(true); echo "2: ".($end - $start)."<br />\n"; unset($start, $end); $start = microtime(true); for($i = 0; $i < 100000000; $i++) if(5 === 10) {} $end = microtime(true); echo "3: ".($end - $start)."<br />\n"; unset($start, $end); $start = microtime(true); for($i = 0; $i < 100000000; $i++) if('foobar' === 10) {} $end = microtime(true); echo "4: ".($end - $start)."<br />\n"; unset($start, $end); ?>

===================================
Result (depending on hardware configuration):
1: 16.779544115067
2: 21.305675029755
3: 16.345532178879
4: 15.991420030594

[#13] austinderrick2 at gmail dot com [2009-10-03 04:50:02]

As an added note to the guy below, in such a case, use the !== operator like this.

$nkey = array_search($needle, $haystack);

if ($nkey !== false) { ...

The !== and the === compare the "types". So, with this type of comparision, 0 is not the same as the FALSE returned by the array_search array when it can not find a match. :)

Quoted Text:

===================================
Be careful with stuff like

if ($nkey = array_search($needle, $haystack)) { ...

if the returned key is actually the key 0, then the if won't be executed
===================================

[#14] strata_ranger at hotmail dot com [2009-04-04 17:13:23]

Although most programmers are aware of this already, if for whatever reason you need to 'break' out of an if() block (which, unlike switch() is not considered a looping structure) just wrap it in an appropriate looping structure, such as a do-while(false):

<?php do if ($foo) { // Do something first... // Shall we continue with this block, or exit now? if ($abort_if_block) break; // Continue doing something... } while (false); ?>

[#15] Anonymous [2009-04-02 04:32:28]

If you need to do something when a function return FALSE and nothing when it return TRUE you can do it like that :
<?php function call() { return FALSE; } if(call()==TRUE) // or if(call()) { // nothing to do } else { // do something here } ?>

You can also write it like this :
<?php if(!call()==TRUE) // or if(!call()) { // do something here } // here '!' will invert 'FALSE' (from call()) into 'TRUE' ?>
/!\ WARNING /!\
The '!' only work with booleans !
Check http://fr.php.net/manual/en/language.types.boolean.php to know if you can use '!'

If you want to compare two strings and use '!' be careful how you use it !!!!
<?php $string1 = "cake"; $string2 = "foo"; if(!$string1==$string2) { echo "cake is a lie"; } //this will ALWAYS fail without exception because '!' is applied to $string1 and not to '$string1==$string2' //to work, you have to do like this if(!($string1==$string2)) { echo "cake is a lie"; } //it will display 'cake is a lie' because ($string1==$string2) return FALSE and '!' will invert it into TRUE ?>
For array/float, it's the same !

[#16] contact at bsorin dot romania [2009-03-07 08:28:14]

This has got the better part of my last 2 hours, so I'm putting it here, maybe it will save someone some time.

I had a

if (function1() && function2())

statement. Before returning true or false, function1() and function2() had to output some text. The trick is that, if function1() returns false, function2() is not called at all. It seems I should have known that, but it slipped my mind.

[#17] Anonymous [2008-09-28 05:03:35]

Re : henryk dot kwak at gmail dot com
<?php function message($m) { echo "$m <br />\r"; return true; } $k=false; if (message("first")&& $k && message("second")){;} // will show //first class $k=true; if (message("first")&& $k && message("second")){;} // will show //first //second ?>

[#18] john [2008-09-24 20:24:59]

@henryk (and everybody):

You should put your arguments in order by *least* likely to be true. That way if php is going to be able to quit checking, it will happen sooner rather than later, and your script will run (what amounts to unnoticeably) faster.

At least, that makes the most sense to me, but I don't claim omniscience.

[#19] chrislabricole at yahoo dot fr [2008-08-09 17:53:51]

You can do IF with this pattern :
<?php $var = TRUE; echo $var==TRUE ? 'TRUE' : 'FALSE'; // get TRUE echo $var==FALSE ? 'TRUE' : 'FALSE'; // get FALSE ?>

[#20] grawity at gmail dot com [2008-03-10 03:41:06]

re: #80305

Again useful for newbies:

if you need to compare a variable with a value, instead of doing

<?php if ($foo == 3) bar(); ?>

do

<?php if (3 == $foo) bar(); ?>

this way, if you forget a =, it will become

<?php if (3 = $foo) bar(); ?>

and PHP will report an error.

[#21] redrobinuk at aol dot com [2008-01-09 02:54:05]

This is aimed at PHP beginners but many of us do this  Ocasionally...

When writing an if statement that compares two values, remember not to use a single = statement.

eg:
<?php if ($a = $b)      {          print("something");      } ?>
This will assign $a the value $b and output the statement.

To see if $a is exactly equal to $b (value not type) It should be:
<?php      if ($a == $b)      {          print("something");      } ?>
Simple stuff but it can cause havok deep in classes/functions etc...