PHP round time() up (future) to next multiple of 5 minutes

 $now = time();     
 $next_five = ceil($now/300)*300;

This will give you the next round five minutes (always greater or equal the current time).

I think that this is what you need, based on your description.

Try:

$time = round(time() / 300) * 300;

Try this function:

function blockMinutesRound($hour, $minutes = '5', $format = "H:i") {
   $seconds = strtotime($hour);
   $rounded = round($seconds / ($minutes * 60)) * ($minutes * 60);
   return date($format, $rounded);
}

//call 
 blockMinutesRound('20:11');// return 20:10