I need a bit more information - like a short reproducing script. The flock() function itself works fine. Are you also sure you're using PHP 5.0.5?

Yes, Im sure, that I use PHP 5.0.5
As extentions I use also: eaccelerator and memcache. mb eaccelerator occurs error?

code:
$fp = fopen($user_data_file, "r+");
if(!$fp) exit;

flock($fp, LOCK_EX); // then I comment this line, scripts works
fseek($fp,0);

I think it was permission error

It's very possible that other extension affect this, and that it is not Xdebug's fault at all. In your last comment you say that it might have been a permission error, but for that to make sense to me you need to be a bit more clearer. Do you mean it was an error on your site? or what was the full error message? I also didn't see a full reproducable script (which I can copy and paste and works immediately)

Can't reproduce this, nor did I receive any feedback whether this still is an issue. Please reopen when you can provide the requested information.