make sure you know this fseek in SplFileObject is different from \fseek.
Argument for fseek in SplFileObject is the line number, not the bytes.
Example:
<?php
$fp = new SplFileObject('./fseek.txt', 'rb');
$fp->fseek(70); // now go to line 70, not byte 70
echo $fp->fgets();
?>