PHP5.5で追加されたDateTimeImmutableをPHP5.4以下で使う

DateTimeImmutable.php

<?php

class DateTimeImmutable extends DateTime
{
    protected static $immutable = true;

    public function add($interval)
    {
        return $this->call('add', func_get_args());
    }

    public function modify($modify)
    {
        return $this->call('modify', func_get_args());
    }

    public function setDate($year, $month, $day)
    {
        return $this->call('setDate', func_get_args());
    }

    public function setISODate($year, $week, $day = 1)
    {
        return $this->call('setISODate', func_get_args());
    }

    public function setTime($hour, $minute, $second = 0)
    {
        return $this->call('setTime', func_get_args());
    }

    public function setTimestamp($unixtimestamp)
    {
        return $this->call('setTimestamp', func_get_args());
    }

    public function setTimezone($timezone)
    {
        return $this->call('setTimezone', func_get_args());
    }

    public function sub($interval)
    {
        return $this->call('sub', func_get_args());
    }

    protected function call($method, array $params)
    {
        if (self::$immutable) {
            self::$immutable = false;
            return call_user_func_array(array(clone $this, $method), $params);
        } else {
            self::$immutable = true;
            return call_user_func_array(array('parent', $method), $params);
        }
    }
}