# function_exists

(PHP 4, PHP 5)

function_exists — Return `TRUE` if the given function has been defined

### Description

bool function_exists ( string `$function_name` )

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

### Parameters

`function_name`

  The function name, as a string.

### Return Values

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().

### 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()
- is_callable()
- get_defined_functions()