GraphQL\Language\Parser::parseOperationType PHP Method

    function parseOperationType()
    {
        $operationToken = $this->expect(Token::NAME);
        switch ($operationToken->value) {
            case 'query':
                return 'query';
            case 'mutation':
                return 'mutation';
                // Note: subscription is an experimental non-spec addition.
            // Note: subscription is an experimental non-spec addition.
            case 'subscription':
                return 'subscription';
        }
        throw $this->unexpected($operationToken);
    }