function_exists

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.

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).

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

method_exists() -

is_callable() -

get_defined_functions() -

class_exists() -

extension_loaded() -