<?php

	echo time_to_iso8601_duration(strtotime("2 years 1 hour 30 minutes", 0));

	function time_to_iso8601_duration($time) {
		$units = array(
			"Y" => 365*24*3600,
			"D" =>     24*3600,
			"H" =>        3600,
			"M" =>          60,
			"S" =>           1,
		);

		$str = "P";
		$istime = false;

		foreach ($units as $unitName => &$unit) {
			$quot  = intval($time / $unit);
			$time -= $quot * $unit;
			$unit  = $quot;
			if ($unit > 0) {
				if (!$istime && in_array($unitName, array("H", "M", "S"))) { // There may be a better way to do this
					$str .= "T";
					$istime = true;
				}
				$str .= strval($unit) . $unitName;
			}
		}

		return $str;
	}

?>

P2YT1H30M