function_exists

Return &true; if the given function has been defined

&reftitle.description; `bool` `function_exists` `string` `function_name`

Checks the list of defined functions, both built-in (internal) and user-defined, for `function_name`.

&reftitle.parameters;

`function_name`

- The function name, as a string.

&reftitle.returnvalues;

Returns &true; if `function_name` exists and is a function, &false; otherwise.

This function will return &false; for constructs, such as `include_once` and `echo`.

&reftitle.examples;

### `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";
    }
    ?>

&reftitle.notes;

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).

&reftitle.seealso;

- `method_exists`
- `is_callable`
- `get_defined_functions`
- `class_exists`
- `extension_loaded`