- floor: documentation ( source)
- date: documentation ( source)
<?php
$d = 1450283411;
echo "Midnight of $d\n";
$date = new DateTime();
$date->setTimestamp($d);
$date->setTime(0,0,0);
echo $date->format('Y-m-d, H:i:s');
echo "\nor\n";
echo date(floor($d/86400)*86400,'Y-m-d H:i:s');