Odd Results when replacing and writing to file.

[OsirisX]

I am building a weak chat system, more like a tag board, and it has a simple user system. When the user loads the tagboard, it reads the cookie and writes their username to file, and when they log out, it loads the users.txt file and replaces their username with a null string. But whenever I log out, it writes to file a seemingly random number less than 10.

<?
//Log out
if($un!=""){
$f=readfile("users.txt");
$f=str_replace("$un","",$f);
$fID=fopen("users.txt","w");
fwrite($fID,$f);
fclose($fID);
}
?>

[the_Igel]

from the manual:

int readfile ( string filename [, int use_include_path])

Reads a file and writes it to standard output.

Returns the number of bytes read from the file. If an error occurs, FALSE is returned and unless the function was called as @readfile, an error message is printed.

Isn't it an answer?

[the_Igel]

from the manual:

int readfile ( string filename [, int use_include_path])

Reads a file and writes it to standard output.

Returns the number of bytes read from the file. If an error occurs, FALSE is returned and unless the function was called as @readfile, an error message is printed.

Isn't it an answer?

[OsirisX]

It is an int? I want to read a file into a string.
Thank you very much.