$epoch = mktime( 0, 0, 0, $startArry['month'], $startArry['day'], $startArry['year'] ) -
mktime( 0, 0, 0, $endArry['month'], $endArry['day'], $endArry['year'] );
printf( "Days : %d", round( $epoch/60/60/24 ) );
printf( "Weeks: %d", round( $epoch/60/60/24/7 ) );
printf( "Months : %d", round( $epoch/60/60/24/30 ) ); // Rounded to 30 days in a month
printf( "Years : %d", round( $epoch/60/60/24/365 ) );
etc, note when working it out if you want months+days, floor the result and deduct it from the epoch