This example is useles, as we don't know which encoding you are using.
Please put this script in a zip file and provide the link to it in this bugreport.

It was tested with iso-8859-2 and windows-1250 text file.
http://republika.pl/gruszczol1/fgetcsv_bug_example.tgz

Specific to 5.X

Try setting LC_CTYPE (or LANG) environment variable to 
"pl_PL.ISO-8859-2".

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

it looks that fgetcsv depends on locales

<?php
$argv[1] = './x.txt';

if (!($fd = fopen($argv[1], 'r')))
        die('Can\'t open file');

while (!feof($fd))
        echo fgets($fd, 4096);

fclose($fd);

if (!($fd = fopen($argv[1], 'r')))
        die('Can\'t open file');

while (($l = fgetcsv($fd, 4096, "\t")))
        print_r($l);

fclose($fd);
?>

x.txt get here http://lampa.naut.cz/x.txt (text contains text windows-1250 encoding "aa?\t?vvv" \t is TAB)

result when locale POSIX:
aa?    ?vvv
Array
(
    [0] => aa?
    [1] => vvv
)

result when locale cs_CZ:
aa?    ?vvv
Array
(
    [0] => aa?
    [1] => ?vvv
)

i tested in on php 5.0.5

I can confirm that this still occurs on PHP 5.1.6 on Linux with windows-1250 text files. Please open it again.

Thanks moriyoshi,
we can do it by changing LC_CTYPE vriable.
we can read unknown char. from csv if it's begining of value,too

Thank you very much