if

(PHP 4, PHP 5, PHP 7)

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。

User Contributed Notes

johovich at yandex dot ru 07-Jul-2017 08:55
'IF' STATEMENT WRONG BEHAVIOR
If assign var to function that returns 0 as integer or 0 as string 'if' statement condition works as false.

Test script:
---------------
//In this script you can see, that set var value to 0 is equal to boolean false
//applied to if statement. There is no diff between integer 0 or string '0'

if($pos = 0){
    $pos++;
} else {
    $pos = 0;
}
var_dump($pos);

//$pos is not boolean false, so it should do if condition true, but it's not

//to make this work well i use this
$pos = 0;
if($pos !== false){
    $pos++;
} else {
    $pos = 0;
}
var_dump($pos);
cole dot trumbo at nospamthnx dot gmail dot com 01-Jun-2017 02:41
Any variables defined inside the if block will be available outside the block. Remember that the if doesn't have its own scope.

<?php
$bool = true;
if ($bool) {
    $hi = 'Hello to all people!';
}
echo $hi;
?>

It will print 'Hello to all people!'

On the other hand, this will have no output:

<?php
if (false) {
    $hi = 'Hello to all people!';
}
echo $hi;
?>
brian at webdesignacademy.co.za 04-Jun-2015 07:25
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
?>
robk 20-May-2013 10:02
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 ?>
sofwan at sofwan dot net 28-Feb-2012 06:31
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
?>
Donny Nyamweya 12-Feb-2011 05:30
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 07:58
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 10:39
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 01:41
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 10:14
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 01:50
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
===================================
contact at bsorin dot romania 07-Mar-2009 05:28
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 02:03
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 05:24
@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.
chrislabricole at yahoo dot fr 10-Aug-2008 02:53
You can do IF with this pattern :
<?php
$var = TRUE;
echo $var==TRUE ? 'TRUE' : 'FALSE'; // get TRUE
echo $var==FALSE ? 'TRUE' : 'FALSE'; // get FALSE
?>
grawity at gmail dot com 10-Mar-2008 12:41
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 11:54
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...