PHP Adding 15 minutes to Time value

Your code doesn't work (parse) because you have an extra ) at the end that causes a Parse Error. Count, you have 2 ( and 3 ). It would work fine if you fix that, but strtotime() returns a timestamp, so to get a human readable time use date().

$selectedTime = "9:15:00";
$endTime = strtotime("+15 minutes", strtotime($selectedTime));
echo date('h:i:s', $endTime);

Get an editor that will syntax highlight and show unmatched parentheses, braces, etc.

To just do straight time without any TZ or DST and add 15 minutes (read zerkms comment):

 $endTime = strtotime($selectedTime) + 900;  //900 = 15 min X 60 sec

Still, the ) is the main issue here.

Though you can do this through PHP's time functions, let me introduce you to PHP's DateTime class, which along with it's related classes, really should be in any PHP developer's toolkit.

// note this will set to today's current date since you are not specifying it in your passed parameter. This probably doesn't matter if you are just going to add time to it.
$datetime = DateTime::createFromFormat('g:i:s', $selectedTime);
$datetime->modify('+15 minutes');
echo $datetime->format('g:i:s');

Note that if what you are looking to do is basically provide a 12 or 24 hours clock functionality to which you can add/subtract time and don't actually care about the date, so you want to eliminate possible problems around daylights saving times changes an such I would recommend one of the following formats:

!g:i:s 12-hour format without leading zeroes on hour

!G:i:s 12-hour format with leading zeroes

Note the ! item in format. This would set date component to first day in Linux epoch (1-1-1970)

strtotime returns the current timestamp and date is to format timestamp

  $date=strtotime(date("h:i:sa"))+900;//15*60=900 seconds
  $date=date("h:i:sa",$date);

This will add 15 mins to the current time