Is there any way I can prevent simultaneously multiple downloads from my site with PHP? I mean I want a user can download only 1 file at a time, the second file can be downloaded only after the first one is finished.

Thanx in advance for the help.

You can use php to initiate the download for you, instead of linking directly to your files.
Also used to prevent "file leeching".

Theres a few examples here: http://us2.php.net/header
Look at the user contributed comments.

What you can do either write to a database, or flatfile when a file is being downloaded.

a user clicks to download your file.zip from the php script url:
Your php script will open a stream to the file that needs to be downloaded, then send that same stream to the browser. (using fopen or fsockopen etc. see: http://us2.php.net/fsockopen )
Your php script then needs to send the appropriate headers to the browser, to let it know that this is a file download (not a regular html page). see: http://us2.php.net/header
Your php script then writes to the database, or flat file, that this file is being downloaded by a certain user with certain session id. see: http://us3.php.net/session

When the user is downloading, you can have a flag that says so. The flag can be removed when the download is done.

So what you write to say a db would be.

"insert into current_downloads set session_id = $session_id, file_id = $file_id, status = 'downloading'";

When you're done you just update the status:
"update current_downloads set status = 'done' where session_id = $session_id and file_id = $file_id";

something along those lines...

hope thta helps.