PayPal\Core\PPMessage::isBuiltInType PHP Method

    private function isBuiltInType($typeName)
    {
        static $types = array('string', 'int', 'integer', 'bool', 'boolean', 'float', 'decimal', 'long', 'datetime', 'double');
        return in_array(strtolower($typeName), $types);
    }