PHP file write threading issues

Use flock to grant access to file only for one user at a time.

But don't forget release your file lock by fclose

Update. Consider this code:

<?php
$start = time();
echo 'Started at '.$start.'<br />';
$filename = 'D:\Kindle\books\Brenson_Teryaya_nevinnost__Avtobiografiya_66542.mobi';
$fp = fopen($filename, 'w+') or die('have no access to '.$filename);

if (flock($fp, LOCK_EX)) {
    echo 'File was locked at '.time().'. Granted exclusive access to write<br />';
}
else {
    echo 'File is locked by other user<br />';
}
sleep(3);
flock($fp, LOCK_UN);
echo 'File lock was released at '.time().'<br />';
fclose($fp);
$end = time();
echo 'Finished at '.$end.'<br />';
echo 'Proccessing time '.($end - $start).'<br />';

Run this code twice (it locks file for 3 seconds, so let's consider our manual script run as asynchronous). You will see something like this:

First instance:

• File was locked at 1302788738. Granted exclusive access to write
• File lock was released at 1302788741

Second:

• File was locked at 1302788741. Granted exclusive access to write
• File lock was released at 1302788744

Notice, that second instance waited for first to release file lock.

If it does not comply your requirements, well... try to invent other solution like: user can read file, then he edit one line and save it as temporary, other user saves his temporary file and so on and once you have all users released file lock, you compose new file as patch of all temporary files on each other (use save files mtime to define which file should stratify other one)... Something like this.... maybe... I'm not the expert in this kind of tasks, unfortunately - just my assumption on how you can get this done.