ActionScript 3.0 :: FileReference Upload Failure

I got a FileRefrence instance for a simple upload and it did work for some time in the past, but now I just can't discover what made it stop woking. Can anyone help me find it out? To simplify, I took only the necessary code (not working as the full version). Here are some details:

1. I'm using Flash Player 9 (this can't change because it is a part of a bigger project). My current host machine is a Windows Vista one. The sandbox is set to network access.

2. The FileReference calls a simple php script hosted at an Apache server (Linux). The script permissions are set to 644 (it works for file uploads coming from plain html pages) and I set the folder permission to 777 just to avoid any other source of errors. Here is the simple code for the php script:

<?php// receiving variables$uid = trim($_POST['uid']); // just to check for a post variable that  I must send while uploading (won't work even if I revome it) 
// a file to check if the script is really called
$myfile = fopen("upload.txt", "wb");fputs($myfile, "file upload for uid $uid
");

[code]....

Just for checking I add a Loader instance and used its "load" method with the same "urlSend" variable. It worked and the php script generated the output file...