タイムゾーンを考慮した date time string <-> unix timestamp 相互変換 ref: http://qiita.com/items/1561

file0.php

<?php
/**
 * Usage:
 *
 *  $ php t.php 1324393200
 *      => 2011-12-20 15:00:00
 *  $ php t.php 1324393200 'Asia/Tokyo'
 *      => 2011-12-21 00:00:00
 *  $ php t.php '2011-12-21 00:00:00'
 *      => 1324393200
 *  $ php t.php '2011-12-21 00:00:00' 'UTC'
 *      => 1324425600
 */
if ($argc < 1) {
    exit(255);
}

if (preg_match('/^\d+$/', $argv[1])) {
    $dt = new \DateTime('@' . $argv[1]);
    if (isset($argv[2])) {
        $dt->setTimeZone(new \DateTimeZone($argv[2]));
    }

    echo $dt->format('Y-m-d H:i:s'), PHP_EOL;
} else {
    $dt = new \DateTime($argv[1], new \DateTimeZone(isset($argv[2]) ? $argv[2] : 'Asia/Tokyo'));
    $dt->setTimeZone(new \DateTimeZone('GMT'));
    echo $dt->format('U'), PHP_EOL;
}