code: function() {
      if (this.token === this.tok.T_FUNCTION) {
        this.next();
        return this.ast.useitem.TYPE_FUNCTION;
      } else if (this.token === this.tok.T_CONST) {
        this.next();
        return this.ast.useitem.TYPE_CONST;
      }
      return null;
    }
docs: Reads the namespace type declaration
  ```ebnf
  use_type ::= (T_FUNCTION | T_CONST)?
  ```
  @see https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L335
  @return {String|null} Possible values : function, const