function_exists

(PHP 4, PHP 5)

function_exists — Return TRUE if the given function has been defined

说明

bool function_exists ( string $function_name )

Checks the list of defined functions, both built-in (internal) and user-defined, for function_name.

参数

function_name: The function name, as a string.

返回值

Returns TRUE if function_name exists and is a function, FALSE otherwise.

Note:

This function will return FALSE for constructs, such as include_once() and echo().

范例

Example #1 function_exists() example


<?php
if (function_exists('imap_open')) {
    echo "IMAP functions are available.<br />
";
} else {
    echo "IMAP functions are not available.<br />
";
}
?>

注释

Note:

A function name may exist even if the function itself is unusable due to configuration or compiling options (with the image functions being an example).