Fix php strtotime bug on 32bit server

<?php
class DateHelper{

    /**
     * 在32位操作系统下，超过 2038-01-19 03:14:07 ，会溢出
     * @param unknown $time
     * @return number
     */
    public static function strtotimeUnsafe($time){
        return strtotime($time);
    }

    /**
     * 安全的方式
     * @param unknown $time
     * @return string
     */
    public static function strtotimeSafe($time){
        $date1 = new DateTime('1970-01-01 00:00:00',new DateTimeZone('Europe/London'));
        $date2 = new DateTime($time,new DateTimeZone('Asia/Shanghai'));
        $interval = $date1->diff($date2);
        $days = $interval->format('%r%a days');
        $days = intval($days);
        //Using this method you need to install php-bcmath module
        return bcmul($days, 24*3600);
    }
}