(PHP 4, PHP 5, PHP 7, PHP 8)
function_exists — Return true if the given function has been defined
$function): bool
Checks the list of defined functions, both built-in (internal) and
user-defined, for function.
functionThe function name, as a string.
Returns true if function exists and is a
function, false otherwise.
Note:
This function will return
falsefor 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).