PHP: calculating time span duration

I needed a function to print a preety time span duration. Nothing special, just simple as that. It turned out to be usefull for many things, like machine (or any other) uptime and so on. Return value is something simmilar to

6 days 3 hours 34 minutes 13 seconds

So here goes, the duration() function. It's published here, so I don't have to waste time and reinvent this everytime.

/*
  Input parameter is the UNIX timestamp 
  of the starting date.
  The second parameter is optional -
  It's value is the ending date,
  also UNIX timestamp. If this
  parameter is not given, the 
  default date is current date.
*/
function duration($start,$end=null) {
 $end = is_null($end) ? time() : $end;

 $seconds = $end - $start;
 
 $days = floor($seconds/60/60/24);
 $hours = $seconds/60/60%24;
 $mins = $seconds/60%60;
 $secs = $seconds%60;
 
 $duration='';
 if($days>0) $duration .= "$days days ";
 if($hours>0) $duration .= "$hours hours ";
 if($mins>0) $duration .= "$mins minutes ";
 if($secs>0) $duration .= "$secs seconds ";
 
 $duration = trim($duration);
 if($duration==null) $duration = '0 seconds';
 
 return $duration;
}