Relative Times in PHP

While working on a side project I found myself in need of a function to produce relative times and dates--e.g. "3 minutes ago" instead of "October 28, 2008 12:18 AM". You'd think this should be easy to find in the age of Google, but I was quite disappointed. It turns out that there are some simple functions out there, but none that adequately take into account the new PHP 5 DateTime object (which avoids UNIX timestamps and therefore allows years prior to 1970 and after 2038), and is performant. So here's my solution for posterity:

/* Warning: this is a quick and dirty solution.  I'm sure there 
   are some bugs or better ways of doing it.  Hoever, feel free 
   to use it however you like. */
 
function is_leap_year($year) {
   return ((($year % 100 == 0) && ($year % 400 == 0)) || ($year % 4 == 0));
}
 
/* for relative dates */
$month_lengths = array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
$units = array("year", "month", "day", "hour", "minute", "second");
$unit_comp_max  = array(12, 30, 24, 60, 60);
 
function relative_time($datetime){
  global $month_lengths;
  global $units;
  global $unit_component_max;
 
  $a = date_parse($datetime);
  $b = date_parse(date("r"));
 
  for ($i=0; $i    $val = 0;
    $unit = $units[$i];
    $val = $b[$unit] - $a[$unit];
    if ($val > 0) {
      if ($a[$units[$i+1]] >= $b[$units[$i+1]]) {
        $val = $val - 1;
        if ($unit == "month") {
          $b['day'] = $b['day'] +$month_lengths[$a['month']];
          if ($a['month'] == 2 && is_leap_year($a['year'])) {
            $b['day'] = $b['day'] + 1;
          }
        } else {
            $b[$units[$i+1]] = $b[$units[$i+1]] + $unit_comp_max[$i];
        }
      }
      if ($val > 0) {
        return  $val . " " . $unit . (($val>1)? "s ago":" ago");
      }
    }
  }
 
  $val = $b['second'] - $a['second'];
  if ($val > 0) {
    return  $val . " " . (($val>1)? "seconds ago":"second ago");
  }

  return "only moments ago";
} 

Please let me know if you have a better solution, but this seems to work for me at the moment.