Date difference output to 0 on 12months

I need to find the exact number of months difference given 2 dates but after it reaches the 12 months, the DateTime class resets itself to zero.

$currDate  = new DateTime('2012-07-31');
$eventDate = new DateTime('2011-07-24');
$interval  = $currDate->diff( $eventDate );
echo $interval->m; // should be 12?

How do I do it such that it continues to count upwards 12, 13, 14…etc?

The DateInterval object is not resetting. It’s containing 1 years 0 months as the interval.

echo $interval->y * 12 + $interval->m;