Using php make a tiny is a joke.

index.js

<?php

// php make compiler is a joke.
class Compiler
{
    // `tokens` 数组用于存所有的 token，待会我们会把 token 一个个 push 进去
    protected $tokens = [];

    // `current` 变量用于存当前处理到哪里了, 就像手指指着纸上我们处理到哪个位置了
    protected $current;

    protected $ast = [
        'type' => 'Program',
        'body' => []
    ];

    public function tokenizer($input) {
        $this->current = 0;

        while ($this->current < strlen($input)) {
            // 把当前的单个字符拿到
            $char = $input[$this->current];
            // 看看是不是左括号
            if ($char == '(') {
                $this->tokens[] = [
                    'type' => 'paren',
                    'value' => '('
                ];

                // 直接下一次循环
                $this->current ++;
                continue;
            }
            // 看看是不是右括号
            if ($char == ')') {
                $this->tokens[] = [
                    'type' => 'paren',
                    'value' => ')'
                ];

                // 直接下一次循环
                $this->current ++;
                continue;
            }
            // 跳过空格,换行符 /\s/
            if (preg_match('/\s/', $char)) {
                $this->current ++;
                continue;
            }
            // 一整串数字
            if (preg_match('/\d/', $char)) {
                $value = '';

                while (preg_match('/\d/', $char)) {
                    $value .= $char;
                    $char = $input[++ $this->current];
                }

                $this->tokens[] = [
                    'type' => 'number',
                    'value' => $value
                ];

                continue;
            }
            // 一整串字符
            if (preg_match('/[a-z]/', $char)) {
                $value = '';

                while (preg_match('/[a-z]/', $char)) {
                    $value .= $char;
                    $char = $input[++ $this->current];
                }

                $this->tokens[] = [
                    'type' => 'name',
                    'value' => $value
                ];

                continue;
            }
            // 异常
            throw new Exception("I dont know what this character is: {$char}");
        }

        return $this->tokens;
    }

    public function parser($tokens = null) {
        $tokens = $tokens ?: $this->tokens;

        $this->current = 0;

        while ($this->current < count($tokens)) {
            $this->ast['body'][] = $this->walk($tokens);
        }

        return $this->ast;
    }

    public function transformer($ast)
    {
        $ast = $ast ?: $this->ast;

        $newAst = [
            'type' => 'Program',
            'body' => []
        ];
    }

    protected function walk($tokens) {
        $token = $tokens[$this->current];

        // 如果是数字
        if ($token['type'] == 'number') {
            $this->current++;

            return [
                'type' => 'NumberLiteral',
                'value' => $token['value']
            ];
        }
        // 如果是括号 而且是 左括号
        if ($token['type'] == 'paren' && $token['value'] == '(') {
            // current + 1 因为在 AST 里我们不关心括号
            $token = $tokens[++ $this->current];
            // 我们创建一个类型是 CallExpression 的节点
            // 值是函数名, 也就是紧跟左括号后面的 add 或 subtract 这样的名字
            $node = [
                'type' => 'CallExpression',
                'name' => $token['value'], // add or subtract, you get the idea
                'params' => []
            ];
            // 函数名处理了, 这里就 +1 跳过函数名.
            $token = $tokens[++ $this->current];

            // 这里用 while 循环, 直到碰到右括号才退出.
            while (($token['type'] != 'paren') ||
                ($token['type'] == 'paren' && $token['value'] != ')')) {
                $node['params'][] = $this->walk($tokens);
                $token = $tokens[$this->current];
            }

            $this->current ++;
            return $node;
        }

        throw new Exception("I can't parse this token {$token}");
    }
}