上一个回答没考虑年份差（不过题主强调了只需要月份），这里补上 @ 2014-03-27 10:06:25：

$time_begin   = strtotime("2012-07-03");
$time_end     = strtotime("2014-03-12");
$time_differ  = $time_end - $time_begin;
$year_differ  = date('Y', $time_differ);
$month_differ = date('m', $time_differ);
$result       = 12*intval($year_differ-1970)+intval($month_differ)-1;

$time_begin   = strtotime("2013-07-03");
$time_end     = strtotime("2014-03-12");
$time_differ  = $time_end - $time_begin;
$month_differ = date('m', $time_differ);
$result       = intval($month_differ)-1;

亲测可行。

    $d1 = new DateTime('2013-07-03');
    $d2 = new DateTime('2014-03-12');
    $diff=$d2->diff($d1);
    echo ($diff->y*12)+$diff->m;