- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$chat_session = "1449675645";
$ts = "15:40:50";
// 5 segundos de diferencia
$datetime = date('Y-m-d', $chat_session) . " " . $ts;
//date_default_timezone_set('GMT');
$datetime_unix = strtotime($datetime);
$segundos = (int)$datetime_unix - (int)$chat_session;
echo "inicio : $chat_session\n";
echo "respuesta : $datetime_unix\n";
echo "diferencia seg : $segundos";