<?php

$d1 = DateTime::createFromFormat('Y-m-d', '2014-03-11');
$d2 = DateTime::createFromFormat('Y-m-d', '2014-04-01');

printf("Whole months difference: %d\n", $d2->diff($d1)->format('n'));
printf("Indexes  difference: %d\n", $d2->format('n') - $d1->format('n'));