day-diff.php

<?php

// takes a start and end date as a string and returns the number of days
// between them
function diffInDays($start, $end)
{
    $start = new DateTime($start);
    $end = new DateTime($end);
    return date_diff($start, $end)->days;
}

diffInDays('today', 'dec 25'); // 191
diffInDays('2015-01-24', '2015-01-24'); // 0
diffInDays('2015-01-24', '2015-02-07'); // 14
diffInDays('2015-01-24', '2015-05-13'); // 109
diffInDays('2015-01-24', '2016-04-24'); // 456

Thanks Zach