Question

Why do we need to check function_exists for user defined functions? It looks ok for internal or core PHP functions but if user know and defined a function himself then why do need to check for its existance?

Below is custom user defined function

if( !function_exists( 'bia_register_menu' ) ) {
    function bia_register_menu() {
        register_nav_menu('primary-menu', __('Primary Menu'));
    }
    add_action('init', 'bia_register_menu');
}

Thanks

Answer 1

To make sure you don’t register the same function twice, which will cause an error.

You also use if(function_exists('function_name')) when you are calling functions defined in plugins. In case you deactivated your plugin, your site will still be functional.