Form empty itself without doing anything - php

Question

Earlier today I asked a question about what I was doing wrong, I got this working but now I'm running into another problem with this script.
Previously
The code gave a warning(), that's fixed now. You can read the post Here
What's the problem?
The code automatically empty itself. When you refresh the page where the script is the text file is empty. I have no idea why...
This is the code
<?php
$fn = "file.txt";
$file = fopen($fn, "w+");
$size = filesize($fn);
if($_POST['addition']) fwrite($file, $_POST['addition']);
fclose($file);
?>
<form action="<?=$PHP_SELF?>" method="post">
<input type="text" name="addition" value="<?php echo file_get_contents('file.txt');?>"/>
<input type="submit"/>
</form>
I use this script to display a youtube video on my website, so I got to update it often.
You can find a working example of the script with this link: http://beta.martijnmelchers.nl/private/Test/test.php
What have i tried?
I didn't try many because I couldn't find a solution for this on the internet and also not in the code.
Please help me again! Thanks in advance!

Answer 1

According to the manual with the w+ option:
Open for reading and writing; place the file pointer at the beginning
of the file and truncate the file to zero length. If the file does not
exist, attempt to create it.
It looks like you want to replace all contents when a post is made, so the easiest solution is to put all file-handling calls in the POST condition:
// To avoid warnings, this is better.
// You can add your original condition after it if you need it.
if ($_SERVER['REQUEST_METHOD'] === 'POST')
{
$file = fopen($fn, "w+");
// not sure why you need this...
// $size = filesize($fn);
fwrite($file, $_POST['addition']);
fclose($file);
}