Round minute down to nearest quarter hour

$seconds = time();
$rounded_seconds = round($seconds / (15 * 60)) * (15 * 60);

echo "Original: " . date('H:i', $seconds) . "\n";
echo "Rounded: " . date('H:i', $rounded_seconds) . "\n";

This example gets the current time and rounds it to the nearest quarter and prints both the original and the rounded time.

PS: If you want to round it down replace round() with floor().

Your full function would be something like this...

function roundToQuarterHour($timestring) {
    $minutes = date('i', strtotime($timestring));
    return $minutes - ($minutes % 15);
}

$now = getdate();
$minutes = $now['minutes'] - $now['minutes']%15;

 //Can add this to go to the nearest 15min interval (up or down)
  $rmin  = $now['minutes']%15;
  if ($rmin > 7){
    $minutes = $now['minutes'] + (15-$rmin);
   }else{
      $minutes = $now['minutes'] - $rmin;
  }

$rounded = $now['hours'].":".$minutes;
echo $rounded;

To round nearest quarter hour use below code

<?php
$time = strtotime("01:08");
echo $time.'<br />';
$round = 15*60;
$rounded = round($time / $round) * $round;
echo date("H:i", $rounded);
?>

01:08 become 01:15