PHP - Manual: if

2024-11-15

if

(PHP 4, PHP 5, PHP 7, PHP 8)

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。

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User Contributed Notes 6 notes

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212

robk ¶

9 years ago


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 ?>

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techguy14 at gmail dot com ¶

11 years ago


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!

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Christian L. ¶

11 years ago


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.

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grawity at gmail dot com ¶

14 years ago


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.

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cole dot trumbo at nospamthnx dot gmail dot com ¶

5 years ago


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;
?>

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Donny Nyamweya ¶

11 years ago


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')

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User Contributed Notes 6 notes