fwrite写NUL

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?

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');

?>

You can use this code, a simplified version, but am not sure if it's the best:

<?php
$fr = fopen("count.txt", "r");
$text = fread($fr, filesize("count.txt"));
$fw = fopen("count.txt", "w");
$text++;
fwrite($fw, $text);
?>

The thing you are missing is rewind(). Without it, after you truncate to 0 bytes, the pointer is still not at the beginning (reference). So when you write your new value, it pads it with NULL in your file.

This script will read a file (or create if it doesn't exist) for a current count, increments, and writes it back to the same file every time the page loads.

$filename = date('Y-m-d').".txt";

$fp = fopen($filename, "c+"); 
if (flock($fp, LOCK_EX)) {
    $number = intval(fread($fp, filesize($filename)));
    $number++;

    ftruncate($fp, 0);    // Clear the file
    rewind($fp);          // Move pointer to the beginning
    fwrite($fp, $number); // Write incremented number
    fflush($fp);          // Write any buffered output
    flock($fp, LOCK_UN);  // Unlock the file
}
fclose($fp);