FIle doesn't get updated by appending text in PHP

I'm trying to append text with PHP to my text file, but 123.txt is still empty after the code execution.

Code:

<?php
$fp = fopen('123.txt', "a+");
fwrite($fp, 'Cats chase dogs');
fclose($fp);
?>

I don't see anything wrong and I don't understand why it's not writing.

Any kind of help would be highly appreciated.

from FIle doesn't get updated by appending text in PHP