php 獲取本月 上月 下一月

<?
 function getMonth($date){
     $firstday = date("Y-m-01",strtotime($date));
      $lastday = date("Y-m-d",strtotime("$firstday +1 month -1 day"));
     return array($firstday,$lastday); 
	  }
	  
	 function getlastMonthDays($date){
     $timestamp=strtotime($date);
     $firstday=date('Y-m-01',strtotime(date('Y',$timestamp).'-'.(date('m',$timestamp)-1).'-01'));
     $lastday=date('Y-m-d',strtotime("$firstday +1 month -1 day"));
     return array($firstday,$lastday);
 }
 
 function getNextMonthDays($date){
    $timestamp=strtotime($date);
    $arr=getdate($timestamp);
    if($arr['mon'] == 12){
        $year=$arr['year'] +1;
        $month=$arr['mon'] -11;
        $firstday=$year.'-0'.$month.'-01';
        $lastday=date('Y-m-d',strtotime("$firstday +1 month -1 day"));
    }else{
        $firstday=date('Y-m-01',strtotime(date('Y',$timestamp).'-'.(date('m',$timestamp)+1).'-01'));
        $lastday=date('Y-m-d',strtotime("$firstday +1 month -1 day"));
    }
    return array($firstday,$lastday);
}

print_r(getMonth("2020-04-21")) ;
print_r(getlastMonthDays("2020-04-21")) ;
print_r(getNextMonthDays("2020-04-21")) ;
?>