The easiest way without reinventing the wheel. This'll give you the full months difference. I.e. the below two dates are almost 76 months apart, but the result is 75 months.
date_default_timezone_set('Asia/Tokyo'); // you are required to set a timezone
$date1 = new DateTime('2009-08-12');
$date2 = new DateTime('2003-04-14');
$diff = $date1->diff($date2);
echo (($diff->format('%y') * 12) + $diff->format('%m')) . " full months difference";
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…