哪位兄弟能帮忙注解一下这段程序？该怎么处理

function BigEndian2Int($byte_word, $signed = false) {
 
        $int_value = 0;
        $byte_wordlen = strlen($byte_word);
        
        for ($i = 0; $i < $byte_wordlen; $i++) {
            $int_value += ord($byte_word{$i}) * pow(256, ($byte_wordlen - 1 - $i));
        }
        
        if ($signed) {
            $sign_mask_bit = 0x80 << (8 * ($byte_wordlen - 1));
            if ($int_value & $sign_mask_bit) {
                $int_value = 0 - ($int_value & ($sign_mask_bit - 1));
            }
        }
 
        return $int_value;
}
 
//获得视频的数字时间
    function getTime($name){
        if(!file_exists($name)){
        return;
    }
    $flv_data_length=filesize($name);
    $fp = @fopen($name, 'rb');
    $flv_header = fread($fp, 5);
    fseek($fp, 5, SEEK_SET);
    $frame_size_data_length =BigEndian2Int(fread($fp, 4));

    $flv_header_frame_length = 9;
    if ($frame_size_data_length > $flv_header_frame_length) {
       fseek($fp, $frame_size_data_length - $flv_header_frame_length, SEEK_CUR);
    }
    $duration = 0;
    while ((ftell($fp) + 1) < $flv_data_length) {
         $this_tag_header     = fread($fp, 16);
         $data_length         = BigEndian2Int(substr($this_tag_header, 5, 3));
         $timestamp           = BigEndian2Int(substr($this_tag_header, 8, 3));
         $next_offset         = ftell($fp) - 1 + $data_length;
         if ($timestamp > $duration) {
          $duration = $timestamp;
         }
 
         fseek($fp, $next_offset, SEEK_SET);
    }
 
    fclose($fp);
    return $duration;
    }
    //转化为0：03：56的时间格式
    function fn($time){
        $num = $time;
        $sec = intval($num/1000);
        $h = intval($sec/3600);
        $m = intval(($sec%3600)/60);
        $s = intval(($sec%60));
        $tm = $h.':'.$m.':'.$s;
        return $tm;
     
    }
 
$t = getTime("/tmp/907701336.flv");//显示数字时间如236722 
echo fn($t);//显示时间格式0:03:56