Convert youtube Api v3 video duration in php

how do i convert PT2H34M25S to 2:34:25

I searched and used this code. I'm new to regex can someone explain this ? and help me with my issue

function covtime($youtube_time){        preg_match_all('/(\d+)/',$youtube_time,$parts);        $hours = floor($parts[0][0]/60);        $minutes = $parts[0][0]%60;        $seconds = $parts[0][1];        if($hours != 0)            return $hours.':'.$minutes.':'.$seconds;        else            return $minutes.':'.$seconds;    }   

but this code only give me HH:MM

so dumb found the solution :

   function covtime($youtube_time){            preg_match_all('/(\d+)/',$youtube_time,$parts);            $hours = $parts[0][0];            $minutes = $parts[0][1];            $seconds = $parts[0][2];            if($seconds != 0)                return $hours.':'.$minutes.':'.$seconds;            else                return $hours.':'.$minutes;        }