DanielKassner.com

<?php
if (!function_exists('calculate_age')) {
	// Checked this function against http://www.timeanddate.com/date/duration.html

	function calculate_age($startdate = '0000-00-00', $enddate = '0000-00-00') {
		
		// Check to see if start date is valid

		@list($year, $month, $day) = explode('-', $startdate);
		if (@!checkdate($month,$day,$year)) {
			return false;
		}
		
		// Create a new datetime object to do our calculations with
		$s = new DateTime($startdate);
		// We have to have an end date so if it is empty or 0000-00-00 we will make it right now
		if (empty($enddate) OR $enddate == '0000-00-00') {
			$enddate = 'now';
		} else {
			// Check to see if end date is valid

			@list($year, $month, $day) = explode('-', $enddate);
			if (@!checkdate($month,$day,$year)) {
				return false;
			}
		}
		// Create a new datetime object to compare as an end date
		$e = new DateTime($enddate);
		
		if ($s->format('Ymd') > $e->format('Ymd')) {
			// End date cannot be before start date
			return false;
		}
		
		// Calculate our base numbers here
		$years = $e->format('Y')-$s->format('Y');
		$months = $e->format('m')-$s->format('m');
		$days = $e->format('d')-$s->format('d');

		// We cannot have negative days
		// Calculate how many days are in the startdate month
		// subtract the startdate day and add 1
		// Then add back in the day of the enddate
		// Subtract 1 month
		if ($days < 0) {
			$days = $s->format('t')-$s->format('d')+1;
			$days += $e->format('d');
			$months--;
		}
		
		// We cannot have negative months
		// Subtract 1 year and add back 12 months to make months positive
		if ($months < 0) {
			$years--;
			$months += 12;
		}

		return array('years'=>$years,'months'=>$months, 'days'=>$days);
	}
}
?>

<?php
// Current date is 2010-01-19
$birthday = '1987-09-23';
$age = calculate_age($birthday);
var_dump($age);
echo '<hr />';
$startdate = '1985-09-06';
$enddate = '1987-09-23';
$difference = calculate_age($startdate, $enddate);
var_dump($difference);
?>