When we create a custom function and understand the usage of variable functions, in order to ensure that the function called by the program exists, we often use function_exists to determine whether the function exists. The same method_exists can be used to detect whether a class method exists.

<?php
function func() {}if (function_exists('func')){   echo 'exists';}
?>

Whether the class is defined can use class_exists.

<?php
class MyClass{}// 使用前檢查類是否存在if (class_exists('MyClass')) {   $myclass = new MyClass();}
?>

There are many such checking methods in PHP, such as whether the file exists file_exists, etc.

<?php
$filename = 'test.txt';if (!file_exists($filename)) {   echo $filename . ' not exists.';}
?>