php时间转换，从a时区转换到b时区

<?php
/**
 * Created by PhpStorm.
 * User: lxd
 * Date: 2019/1/9
 * Time: 13:25
 * Comm:
 */

class DtFormat
{
    protected $dt;
    protected $ttz; //目标时区
    protected static $instance;

    private function __construct( $ftz, $ttz )
    {
        if( !$ftz || !$ftz instanceof DateTimeZone )
            $ftz        = new DateTimeZone( date_default_timezone_get() ); //默认来源时区

        if( !$ttz || !$ttz instanceof  DateTimeZone )
            $this->ttz  = new DateTimeZone( C('PARTNER_TIME_ZONE') ); //默认输出时区
        else
            $this->ttz  = $ttz;

        $this->dt   = new DateTime( 'now', $ftz );
    }

    /**
     * author liuxiaodong
     * date 2019/1/11 15:19
     * @param null $ftz 来源时区
     * @param null $ttz 目标时区
     * @return DtFormat
     */
    public static function getInstance( $ftz = null, $ttz = null )
    {
        if( !$ftz && !$ttz )
            $key    = '0';
        else {
            $ftz_key    = $ftz instanceof DateTimeZone ? $ftz->getName() : null;
            $ttz_key    = $ttz instanceof DateTimeZone ? $ttz->getName() : null;
            $key        = md5( $ftz_key . $ttz_key );
        }
        if( !self::$instance[$key] instanceof self ) {
            self::$instance[$key]     = new self( $ftz, $ttz );
        }

        return self::$instance[$key];
    }

    public function show( $original, $formart = 'Y-m-d H:i:s' )
    {
        //将时间转成时间戳
        $new_original       = @($original + 0);
        if (strlen($new_original) != strlen($original)) {
            $new_original   = strtotime($original);
            if (!$new_original)
                return $original;
        }

        /**
         * notice
         * 时区不一致，同一个时间转换成时间戳是不一样的 ！！
         *  $str    = '2018-09-26 07:23:31';
            var_dump( date_default_timezone_get() ); //UTC
            var_dump( strtotime( $str ) ); //1537946611

            date_default_timezone_set( 'PRC' );
            var_dump( strtotime( $str ) ); //1537917811
         *
         */
        $datetime       = null;
        if( $this->dt->getTimezone()->getName() == date_default_timezone_get() ) {
            $datetime   = clone $this->dt;
            $datetime   = $datetime->setTimestamp($new_original);
        }else {
            $tmpDt      = new DateTime();
            $tmpDt->setTimestamp( $new_original );
            $datetime   = DateTime::createFromFormat( 'Y-m-d H:i:s', $tmpDt->format( 'Y-m-d H:i:s' ), $datetime->getTimezone() );

 } $datetime->setTimezone($this->ttz); return $datetime->format( $formart ); } }

 调用：　

try{
            $timestr         = '2018-10-10 12:00:00';
            $ftimezone       = new DateTimeZone( 'UTC' );
            $ttimezone       = new DateTimeZone( 'PRC' );
            $newtm           = DtFormat::getInstance( $ftimezone, $ttimezone )->show( $timestr );
            echo(  "old = {$timestr}; new = {$newtm}" );
        }catch ( Exception $e ) {
            //error
        }

结果：　　old = 2018-10-10 12:00:00; new = 2018-10-10 20:00:00