private function splitVariableDefault($variable_name, $parameter_symbol)
    {
        $variable_default = explode($parameter_symbol, $variable_name, 2);

        if (count($variable_default) !== 2 || empty($variable_default[1])) {
            throw new ParseException(
                'You must have valid parameter expansion syntax, eg. ${parameter:=word}',
                $variable_name,
                $this->parser->line_num
            );
        }

        return array(trim($variable_default[0]), trim($variable_default[1]));
    }