PHP Manual
Function handling Functions
Return true if the given function has been defined

function_exists

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

function_exists — Return true if the given function has been defined

Description

function_exists(string $function): bool

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

Parameters

function: The function name, as a string.

Return Values

Returns true if function exists and is a function, false otherwise.

Note:
This function will return false for constructs, such as include_once and echo.

Examples

Example #1 function_exists() example

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

Notes

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).

See Also

method_exists() - Checks if the class method exists
is_callable() - Verify that a value can be called as a function from the current scope.
get_defined_functions() - Returns an array of all defined functions
class_exists() - Checks if the class has been defined
extension_loaded() - Find out whether an extension is loaded