Store number of running scripts in a file

Question

I have a PHP script that runs during about 1s (15s for my tests), and I would like to store the number of threads that are running this script to do a specific action when too many scripts/threads are running.

To do that I tried to store the number of opened tasks in a file.

1. I read the value in the file, increment, and write the new value at the beginning
2. Do my stuff
3. Read the value in the file, decrement, and write the new value at the beginning

Here is my code:

<?php


    define("MAX_THREADS", 5);
    define("LOCK_FILE", "threads.txt");


    $fp = fopen(LOCK_FILE, "r+");
    if (flock($fp, LOCK_EX)) { // acquire an exclusive lock
        $threadNb = fread($fp, filesize(LOCK_FILE));
        /*
        * if $threadNb > MAX_THREAD => Do something
        */
        ftruncate($fp, 0);     // truncate file
        fwrite($fp, ++$threadNb);
        fflush($fp);            // flush output before releasing the lock
        flock($fp, LOCK_UN);    // release the lock
    } else {
        echo "Failed lock the file!";
    }
    fclose($fp);


    // Do stuff

    // ...

    // End of script


    sleep(15);

    $fp = fopen(LOCK_FILE, "r+");
    if (flock($fp, LOCK_EX)) { // acquire an exclusive lock
        $threadNb = fread($fp, filesize(LOCK_FILE));
        ftruncate($fp, 0);     // truncate file
        fwrite($fp, --$threadNb);
        fflush($fp);            // flush output before releasing the lock
        flock($fp, LOCK_UN);    // release the lock
    } else {
        echo "Failed lock the file!";
    }
    fclose($fp);

PROBLEM: The value of $threadNb is always equal to 0!!