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 ?>
(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 = $a;
}
?>
if 语句可以无限层地嵌套在其它
if 语句中，这给程序的不同部分的条件执行提供了充分的弹性。
easy way to execute conditional html / javascript / css / other language code with php if else:<?php if (condition): ?>html code to run if condition is true<?php else: ?>html code to run if condition is false<?php endif ?>Left-to-right evaluation of && operators has one useful feature: evaluation stops after first "false" operand is encountered. This feature can be useful for creating following construction:$someVar==123 is not evaluated, so there will be no warnings such as "Undefined variable $someVar":<?php// $someVar=123; - commented lineif ((!empty($someVar))&&($someVar==123)){ echo $someVar;}?>Function someFunc($someVar) will not be called:<?php// $someVar=123; - commented lineif ((!empty($someVar))&&(someFunc($someVar))){ echo $someVar;}?>This will give "Warning: Undefined variable $someVar" error. Order matters:<?php// $someVar=123;if ((someFunc($someVar))&&(!empty($someVar))){ echo $someVar;}?>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 || $c==6) ) {
//do that something here.
}
?>
Hope this helps!re: #80305Again useful for newbies:if you need to compare a variable with a value, instead of doing<?phpif ($foo == 3) bar();?>do<?phpif (3 == $foo) bar();?>this way, if you forget a =, it will become<?phpif (3 = $foo) bar();?>and PHP will report an error.An other way for controls is the ternary operator (see Comparison Operators) that can be used as follows:<?php$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.