【php】年をまたぐ月数をカウントする

$start_date ='2019-07-01';
$end_date   ='2021-05-31';

$month1   = date('Y', strtotime($start_date)) * 12 + date('m', strtotime($start_date));
$month2   = date('Y', strtotime($end_date)) * 12 + date('m', strtotime($end_date));
$sabun    = $month2 - $month1;

echo $sabun;