控制结构
在线手册：中文 英文
PHP手册

if

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。

控制结构
在线手册：中文 英文
PHP手册
PHP手册 - N: if

用户评论:

marcin_wo_wroc at o2 dot pl (23-Mar-2012 06:26)

Here's something uncommon:
How to make one else statement for two nested if conditions?
aka: how to make one else for two ifs.

By default people simply copy & paste code in else like that:
<?php
if ( is_object($times)){
 if ((float)$times->getTime()>0){
    //code
 }else{
     //else code Alpha
 }
}else{
    //duplicated else code Alpha
}
?>

but it's much better and easier to simply use one condition inside of another, like that:

<?php
if ( is_object($times) ? (float)$times->getTime()>0 : false){
    //put here your code that gonna be executed if $times is an object and if $times->getTime() is greater than zero
    //condition is the same as:
    //if (is_object($times)){
    //    if ((float)$times->getTime()>0){
    //            //code
    //    }
    //}
}else{
    //put here your else statement for conditions above
}
?>

simple & beautiful :)

sofwan at sofwan dot net (28-Feb-2012 05:31)

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
?>

gung foo priest (22-Jan-2012 04:13)

inline if syntax:

echo (True==True) ? 'true' : 'false';

is the same as:

if(True==True) echo 'true';
else echo 'false';

bimal at sanjaal dot com (07-Jul-2011 10:21)

PHP compares numbers inside quotations, in an interesting way.
This can create confusions to those who did not refer this manual, but expected something different.

<?php

define('NUMBER', 13);
$number = NUMBER;

if('13_2' == NUMBER) { echo('Why matched?'); };
if('13_2' == "{$number}") { echo('Why not matched?'); }

?>

In the above example, the first comparison matches; that you don't expect.

The second comparison does not match.

lallemand dot mathieu at gmail dot com (31-Mar-2011 02:01)

Be careful when you chain inline "if" :

<?php
$x = 1;
$y = 3;
echo ($x==1) ? "One" : ($y == 2) ? "Two" : "None";
?>

Expected result : "One".
Result on screen : "Two".

Pretty disapointing isn't it ?

So, if you want to chain inline "if" you have to use parentesis on each test like below:

<?php
$x=1;
$y=3;
echo ($x==1) ? "One" : ( ($y==2) ? "Two" : "None" );
?>

Result on screen : "One".

Hope it helps !

Donny Nyamweya (12-Feb-2011 04:30)

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')

Christian L. (25-Jan-2011 06:58)

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.

techguy14 at gmail dot com (06-Jan-2011 09:39)

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!

admin at leonard !spam challis dot com (22-Nov-2010 12:41)

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".

Rudi (14-Sep-2010 09:14)

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

austinderrick2 at gmail dot com (03-Oct-2009 12:50)

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
===================================

jm+phpweb at roth dot lu (28-Aug-2009 10:25)

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

strata_ranger at hotmail dot com (05-Apr-2009 01:13)

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);
?>

Anonymous (02-Apr-2009 12:32)

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 !

contact at bsorin dot romania (07-Mar-2009 04:28)

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.

Anonymous (28-Sep-2008 01:03)

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 
?>

john (25-Sep-2008 04:24)

@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.

Wiseguy (28-Aug-2008 03:22)

RE: chrislabricole at yahoo dot fr on 09-Aug-2008 05:53

You're referring to the ternary operator.

http://php.net/manual/en/language.operators.comparison.php

jchau at bu dot edu (14-Aug-2008 06:50)

RE: henryk dot kwak at gmail dot com's comment from 04-May-2008 05:01

I think you made a mistake.

For maximum efficiency, assuming each expression requires the same amount of processing, the expression that is least likely to be true should come first for expressions connected by && (and).  This will reduce the probability that later expressions will need to be evaluated. 

The opposite is true for || (or).  If the most likely expression comes first, then the probability of needing to evaluate later expressions is reduced.

chrislabricole at yahoo dot fr (10-Aug-2008 01:53)

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

henryk dot kwak at gmail dot com (05-May-2008 01:01)

When you use if command with many condidions like
if ( expr1 && expr2 && expr3 && etc. )
it is more effective to put expressions in special order
Firstly you should put that, which has the biggest
probability to occur.
This is because PHP checks each condition in order from left to right and it takes some time to check each condition.

grawity at gmail dot com (10-Mar-2008 11:41)

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.

redrobinuk at aol dot com (09-Jan-2008 10:54)

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...