date_default_timezone_set('PRC'); function get_month_last_day($date = '') { if($date != ''){ $time = strtotime($date); }else{ $time = time(); } //获取该日期是当月的第几天 $day = date('j',$time); return date('Y-m-d', strtotime("- {$day} days",$time)); } echo get_month_last_day('2018-03-25');