Use a unix timestamp and the datea and time methods to make an initial time, and then bump it up by how wver emany seconds there is in fifteen minutes (900 I think!)
$time = "place Unix timestamp for a 08:00 here";
for($time; $time < something; $time + 900) {
Do clever stuff here
}
Can't remember my for loop syntax now 🙂
HTH
glj