<?php
$zones_array = array();
$timestamp = time();
# to maintain the original timezone, store before
$default_timezone = date_default_timezone_get();
foreach (timezone_identifiers_list() as $key => $zone) {
    date_default_timezone_set($zone);
    $zones_array[$key]['zone'] = $zone;
    $zones_array[$key]['diff_from_GMT'] = date('P', $timestamp);
}
# to maintain the original timezone, re-set after
date_default_timezone_set($default_timezone);