Multibyte pointer when reading part of a file in PHP - php

Question

I use PHP.
The function below loads part of a big multibyte enter separated CSV file and return a pointer (the end position) and the content in an array. With the pointer I can later do another run. It works:
function part($path, $offset, $rows) {
$buffer = array();
$buffer['content'] = '';
$buffer['pointer'] = array();
$handle = fopen($path, "r");
fseek($handle, $offset);
if( $handle ) {
for( $i = 0; $i < $rows; $i++ ) {
$buffer['content'] .= fgets($handle);
$buffer['pointer'] = mb_strlen($buffer['content']);
}
}
fclose($handle);
return($buffer);
}
// Buffer first part
$buffer = part($path_to_file, 0, 100);
// Buffer second part
$buffer = part($path_to_file, $buffer['pointer'], 100);
print_r($buffer);
If I change the $buffer['pointer'] line to:
$buffer['pointer'] = mb_strlen($buffer['content'], "UTF-8");
...it does not work anymore... I understand that it uses the different encoding when I use UTF-8 instead of the default, but why doesn't it work with UTF-8?
Shouldn't UTF-8 be compatible with foreign characters?
Because the function above works when I use it without "UTF-8" I guess I could just use it without UTF-8.
I'm still worried that in some cases it can give the wrong pointer?
Is there a safer way to get the correct pointer?
Encoding test
When I do this I get UTF-8:
echo mb_detect_encoding($buffer['content']);

Answer 1

This has little to do with UTF-8. Filesystem functions (like fseek(), fread(), etc.) operate on individual bytes. They don't care about the encoding at all. (You could be writing / reading binary data).
If you want to store a pointer to fseek() to at a later time, use ftell() to find out the current position:
$buffer['pointer'] = ftell($handle);