filesize() Problem

Hi here!
I write an guestbook and i have the following problem:
I open and read a file with the following code:

$open=fopen($file, "r");
$size=filesize($file);
$inhalt=fread($open, $size);
fclose($open);

somewhere later in the script I add something to this file and then want to read the file again with the same code I wrote above, but I only get the old (non-added) content.
So I have to restart the script to see the changes.

I think filesize($file); olny works the first time bacause the second time I use it in the script I get the old datas.
Dies anyone know why, or can you help me?

Thanks, Kai from Germany.