there is a bug using fseek on previos versions of PHP8.0.1
See:

https://bugs.php.net/bug.php?id=46569

https://3v4l.org/O89dJ

I solved it with a function:

<?php
/**
* SEEK an Spl object
* There is a bug in php for seeking files
* seems solved php_version > PHP8.0.1
* See https://bugs.php.net/bug.php?id=46569
* & https://3v4l.org/O89dJ
*
* $Spl->seek() Works ok in all versions with offset 0 (first row)
* On PHP_VERSION < 8.0.1:
*  - Offset 1: seek() cannot seek at row 1. It will be done manually, rewind file and reading first row
*  - Rest of Offsets: The cursor remains at next row of $Offset
*
* @param SplObject $Spl
* @param int $Offset
*/

function seek_spl($Spl, $Offset){
    if (version_compare(PHP_VERSION, '8.0.1', '>=') || $Offset == 0) {
        $Spl->seek($Offset);
    } else {
        if( $Offset == 1 ){
            $Spl->rewind(); // Ensure to go at first row before exit
            $Spl->fgets(); // Read line 0. Cursor remains now at line 1
        } else {
            $Spl->seek($Offset-1);
        }
    }
}?>

A sleek way of counting the number of lines in a file can be like below

$file = new \SplFileObject('file.extension', 'r');
$file->seek(PHP_INT_MAX);

echo $file->key() + 1;