Function Existence Check Using PHP
function_exists — Return TRUE if the given function has been defined. To check that whether a function is defined or exists or not use function_exists. 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"; } ?>
