php 获取给定日期的前一天日期

 /**
     * 
     * 获取给定日期的前一天
     * @param string $date
     * @return string $yesterday
     */
    public function getYesterday($date)
    {
        if(empty($date)) 
        {
            $yesterday = date("Y-m-d",strtotime("-1 day"));
        }else{
            $arr = explode('-', $date);
            $year = $arr[0];
            $month = $arr[1];
            $day = $arr[2];
            $unixtime = mktime(0,0,0,$month,$day,$year)-86400;
            $yesterday = date('Y-m-d',$unixtime);
        }
        
        return $yesterday;
    }