获取mobi / azw3的元数据

There is a python script to get metadata from a mobi/azw3 file, I try to get the title/author by php with the code:

$fp=fopen('test.mobi', 'r');
fseek($fp,1);
echo fgets($fp, 503);

where 503 is the position for title (see wiki) it seems not the title. I think I need to set the ending of the steam, but how?