#!/usr/bin/php

<?php

$start_date = "2013-12-02";
$start_time = "06:30:00";
$end_time   = "06:25:00";

$start = $start_date." ".$start_time ;
$end   = $start_date." ".$end_time ;

if ( $start > $end ) {
        $test = new DateTime($end);
        $test->modify(' +1 day');
}

print $test->format('Y-m-d H:i:s');

echo "\n";

?>