(PHP 4, PHP 5)
function_exists — Return TRUE if the given function has been defined
Checks the list of defined functions, both built-in (internal) and user-defined, for function_name .
The function name, as a string.
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().
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";
}
?>
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).