find the closest next hour

$nextHour = (intval($date->format('H'))+1) % 24;
echo $nextHour; // 5

Supply any eligible date() to:

function roundToNextHour($dateString) {
    $date = new DateTime($dateString);
    $minutes = $date->format('i');
    if ($minutes > 0) {
        $date->modify("+1 hour");
        $date->modify('-'.$minutes.' minutes');
    }
    return $date;
}

Here we go:

<?php
    echo date("H:00",strtotime($date. " + 1hour "));
?>

Can you just take pieces (hours, minutes, seconds) and get the next hour?

$dateString = 'Tue, 13 Mar 2012 04:48:34 -0400';
$date = new DateTime( $dateString );

echo $date->format( 'H:i:s' );
echo "\n";

$nexthour = ($date->format('H') + ($date->format('i') > 0 || $date->format('s') > 0 ? 1 : 0)) % 24;
echo "$nexthour:00:00";