<?php
$dt = new DateTime('20151215T073625Z');
$tz = new DateTimeZone('Asia/Kolkata'); // or whatever zone you're after
$dt->setTimezone($tz);
echo $dt->format('Y-m-d H:i:s');
// $d = new DateTime('20100510133046.0Z');
// echo $d->format('Y-m-d H:i:s');