HI,
you can do round(x/15)*15. This give you the nearest quarter...
// this is to find 15 minute timestamp...
$quote=mktime(0,30,0,1,1,0)-mktime(0,15,0,1,1,0);
//get a time
$thistime=mktime(0,8,10,11,12,2003);
//round to the nearest quarter...
$thistime=round($thistime/$quote)*$quote;
I hope this helps...