fwrite writes NUL

Question

I'm trying to write to a file with PHP and this is the code I'm using (taken from this answer to my previous question):

$fp = fopen("counter.txt", "r+");

while(!flock($fp, LOCK_EX)) {  // acquire an exclusive lock
    // waiting to lock the file
}

$counter = intval(fread($fp, filesize("counter.txt")));
$counter++;

ftruncate($fp, 0);      // truncate file
fwrite($fp, $counter);  // set your data
fflush($fp);            // flush output before releasing the lock
flock($fp, LOCK_UN);    // release the lock

fclose($fp);

The read part works fine, if the file gets read, it's content is read well, i.e. if the file contains 2289, then 2289 is read.

The problem is that when it increments and rewrites the value to that file, [NUL][NUL][NUL][NUL][NUL][NUL][NUL][NUL]1 gets written.

What am I missing? Why do null characters get written?

Answer 1

EDIT #2:

Try this with flock (tested)

If file is not locked, it will throw an Exception (see added line) if(...

I borrowed the Exception snippet from this accepted answer.

<?php

$filename = "numbers.txt";
$filename = fopen($filename, 'a') or die("can't open file");

if (!flock($filename, LOCK_EX)) {
    throw new Exception(sprintf('Unable to obtain lock on file: %s', $filename));
}

file_put_contents('numbers.txt', ((int)file_get_contents('numbers.txt'))+1);

// To show the contents of the file, you 
// include("numbers.txt");

    fflush($filename);            // flush output before releasing the lock
    flock($filename, LOCK_UN);    // release the lock


fclose($filename);
echo file_get_contents('numbers.txt');

?>