Overriding DateTime

<?php

namespace BookingNamespace;

/**
 * extend DateTime functionalities so that we are able to define what should be considered to be "now"
 *   - set_now allows to define "now"
 *   - modify_now applies the modify function on what is defined as "now"
 *   - reset is for going back to original DateTime behaviour
 *
 * @author mika
 */
class DateTime extends \DateTime
{
  private static $fake_time=null;

  // public static method that extend DateTime functionalities
  public static function reset(){ self::$fake_time = null; }
  public static function set_now($time = "now", DateTimeZone $timezone = null)
  {
    self::$fake_time = new \DateTime($time, $timezone);
  }
  public static function modify_now($modify){ if( ! is_null(self::$fake_time)) self::$fake_time->modify($modify); }

  // private methods for the constructor
  private function is_virtual(){ return ! is_null(self::$fake_time); }
  private function is_relative_date($val){ return strtotime($val) != strtotime($val, 0); }

  /**
  *   constructor:
  *       - if we work with virtual now, relative times are calculated based on $fake_time
  *       - DateTimeZone is ignored in virtual context (and relative time).
  *       - if we do not work with virtual now, parent constructor is called.
  */
  public function __construct($time = "now", DateTimeZone $timezone = null)
  {
      if($this->is_virtual()&&$this->is_relative_date($time))
      {
          parent::__construct(self::$fake_time->format(DateTime::ISO8601));
          parent::modify($time);
      }
      else
        parent::__construct($time, $timezone);
  }
}

?>