here my conversion:
function add_date($givendate,$hr=2)
{
$cd = strtotime($givendate);
$newdate = date('Y-m-d H:i:s', mktime(date('h',$cd)+$hr, date('i',$cd), date('s',$cd), date('m',$cd), date('d',$cd), date('Y',$cd)));
return $newdate;
}
http://www.symfony-project.org/reference/1_4/en/04-Settings#chapter_04_sub_default_timezone
Default timezone is in settings