Elegant way to get the count of months between two dates?

Or, if you want the procedural style:

$date1 = new DateTime("2009-09-01");
$date2 = new DateTime("2010-05-01");
$interval = date_diff($date1, $date2);
echo $interval->m + ($interval->y * 12) . ' months';

UPDATE: Added the bit of code to account for the years.

Or a simple calculation would give :

$numberOfMonths = abs((date('Y', $endDate) - date('Y', $startDate))*12 + (date('m', $endDate) - date('m', $startDate)))+1;

Accurate and works in all cases.

For PHP >= 5.3

$d1 = new DateTime("2009-09-01");
$d2 = new DateTime("2010-05-01");

var_dump($d1->diff($d2)->m); // int(4)
var_dump($d1->diff($d2)->m + ($d1->diff($d2)->y*12)); // int(8)

DateTime::diff returns a DateInterval object

If you don't run with PHP 5.3 or higher, I guess you'll have to use unix timestamps :

$d1 = "2009-09-01";
$d2 = "2010-05-01";

echo (int)abs((strtotime($d1) - strtotime($d2))/(60*60*24*30)); // 8

But it's not very precise (there isn't always 30 days per month).

Last thing : if those dates come from your database, then use your DBMS to do this job, not PHP.

Edit: This code should be more precise if you can't use DateTime::diff or your RDBMS :

$d1 = strtotime("2009-09-01");
$d2 = strtotime("2010-05-01");
$min_date = min($d1, $d2);
$max_date = max($d1, $d2);
$i = 0;

while (($min_date = strtotime("+1 MONTH", $min_date)) <= $max_date) {
    $i++;
}
echo $i; // 8