Calculate the difference between two dates.

date_difference.php

<?php
/**
 *  Calculate the difference between two dates.
 *
 *  @param   string   $int       The interval to calculate (y = years, m = months, d = days, h = hours, i = minutes, s = seconds)
 *  @param   string   $d1        The first date (YYYYMMDD)
 *  @param   string   $d2        The second date (YYYYMMDD)
 *  @param   mixed    $round     Whether to return a decimal value, or round up or down
 *  @param   boolean  $absolute  Whether the value should always be positive
 *  @return  integer  $results   The calculated value
 */
function date_difference($int, $d1, $d2, $round=true, $absolute=true) {

    // Make sure we are working with strings
    if(is_string($d1))
        $d1 = date_create($d1);
    if(is_string($d2))
        $d2 = date_create($d2);

    // Get the difference
    $diff = date_diff($d1, $d2, $absolute);

    // Decide what we are calculating
    switch($int){
        case 'y':
            $results = $diff->y + $diff->m / 12 + $diff->d / 365.25;
            break;
        case 'm':
            $results= $diff->y * 12 + $diff->m + $diff->d/30 + $diff->h / 24;
            break;
        case 'd':
            $results = $diff->y * 365.25 + $diff->m * 30 + $diff->d + $diff->h/24 + $diff->i / 60;
            break;
        case 'h':
            $results = ($diff->y * 365.25 + $diff->m * 30 + $diff->d) * 24 + $diff->h + $diff->i/60;
            break;
        case 'i':
            $results = (($diff->y * 365.25 + $diff->m * 30 + $diff->d) * 24 + $diff->h) * 60 + $diff->i + $diff->s/60;
            break;
        case 's':
            $results = ((($diff->y * 365.25 + $diff->m * 30 + $diff->d) * 24 + $diff->h) * 60 + $diff->i)*60 + $diff->s;
            break;
    }

    // Round the value if needed
    switch($round) {
        case 'up':
            $results = ceil($results);
            break;
        case 'down':
            $results = floor($results);
            break;
        case true:
            $results = round($results);
            break;
    }

    // Invert the value if needed
    if($diff->invert) {
        return -1 * $results;
    } else {
        return $results;
    }
}