Moment on a Timetable

Write a class Time to represent the time which you often see on a timetable. Your class should implement the following methods:
  1. __construct($string) Creates a time with the given string. The format of the string is "hh:mm", where hh represents hours from 00 to 23, and mm represents minutes from 00 to 59. This constructor should initialize the instance variables $hour and $minute to store the input time.
  2. modify($d) Shifts the time by d minutes. If $d is negative, the time is changed to a previous moment. For example, "13:15" becomes "11:45" after it is shifted -90 minutes.
  3. delta($d) Similar to modify(). However, delta() return an object with the shifted time but not modifying the object itself, in contrast with modify() which modifies the object without returning anything.
  4. getTime() Returns a string that shows the time with the correct format, e.g., "09:10", "23:55".
You may test your class with the following main program. 

<?php
    $departure = new Time("22:54");
    $arrival   = new Time("23:36");
    print "Your train departs at\t\t" . $departure->getTime();
    print " and arrives at" . $arrival->getTime() . "<br />\n";

    $departure->modify(-39);
    $arrival->modify(-39);
    print "The previous train departs at\t\t" . $departure->getTime();
    print " and arrives at" . $arrival->getTime() . "<br />\n";

    $departure->modify(99);
    $arrival->modify(99);
    print "The next train departs at\t\t" . $departure->getTime();
    print " and arrives at" . $arrival->getTime() . "<br />\n";

?>
The result may look like 

Your train departs at            22:54 and arrives at 23:36
The previous train departs at    22:15 and arrives at 22:57
The next train departs at        23:54 and arrives at 00:36