parseArrow example

public function __construct(Parser $parser, Environment $env)
    {
        $this->parser = $parser;
        $this->env = $env;
        $this->unaryOperators = $env->getUnaryOperators();
        $this->binaryOperators = $env->getBinaryOperators();
    }

    public function parseExpression($precedence = 0, $allowArrow = false)
    {
        if ($allowArrow && $arrow = $this->parseArrow()) {
            return $arrow;
        }

        $expr = $this->getPrimary();
        $token = $this->parser->getCurrentToken();
        while ($this->isBinary($token) && $this->binaryOperators[$token->getValue()]['precedence'] >= $precedence) {
            $op = $this->binaryOperators[$token->getValue()];
            $this->parser->getStream()->next();

            if ('is not' === $token->getValue()) {
                $expr = $this->parseNotTestExpression($expr);
            }