PHP Manual: function

function_exists

(PHP 3 >= 3.0.7, 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. Returns TRUE on success or FALSE on failure.

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

Note that 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). Also note that function_exists() will return FALSE for constructs, such as include_once() and echo().