code: function parseKey(expr) {
    switch(expr.kind) {
      case 'string':
        return expr.value
      case 'number':
        return parseInt(expr.value, 10)
      case 'boolean':
        return expr.value ? 1 : 0;
      default:
        throw new Error(`Unexpected PHP key: "${expr.kind}", details: ${JSON.stringify(expr)}`)
    }
  }
docs: Parse a PHP expression to JavaScript
  @param  {Object} expr The AST PHP expression.
  @return {*}           A JavaScript object or value.