Method register

/**
 * Registers a function to be called with a specific key
 *
 * @param   string    $key       The name of the key
 * @param   callable  $function  Function or method
 *
 * @return  boolean  True if the function is callable
 *
 * @since       1.6
 * @deprecated  5.0 Use the service registry instead
 */
public static function register($key, callable $function)
{
    @trigger_error('Support for registering functions is deprecated and will be removed in Joomla 5.0, use the service registry instead', E_USER_DEPRECATED);
    list($key) = static::extract($key);
    static::$registry[$key] = $function;
    return true;
}