function sec2hms ($sec, $padHours = false) {

    $hms = "";
    
    // there are 3600 seconds in an hour, so if we
    // divide total seconds by 3600 and throw away
    // the remainder, we've got the number of hours
    $hours = intval(intval($sec) / 3600); 

    // add to $hms, with a leading 0 if asked for
    $hms .= ($padHours) 
          ? str_pad($hours, 2, "0", STR_PAD_LEFT). ':'
          : $hours. ':';
     
    // dividing the total seconds by 60 will give us
    // the number of minutes, but we're interested in 
    // minutes past the hour: to get that, we need to 
    // divide by 60 again and keep the remainder
    $minutes = intval(($sec / 60) % 60); 

    // then add to $hms (with a leading 0 if needed)
    $hms .= str_pad($minutes, 2, "0", STR_PAD_LEFT). ':';

    // seconds are simple - just divide the total
    // seconds by 60 and keep the remainder
    $seconds = intval($sec % 60); 

    // add to $hms, again with a leading 0 if needed
    $hms .= str_pad($seconds, 2, "0", STR_PAD_LEFT);

    return $hms;
}

Convert seconds to hh:mm:ss

PHP