PHP Flat File .txt Issue "Social Networking Site"

Ok I am only having one problem. Here is the code:

<?php
$myfile = “bio.txt”;
if (isset($_POST[‘ta’])) {
$newData = nl2br(htmlspecialchars($_POST[‘ta’]));
$handle = fopen($myfile, “w”);
fwrite($handle, $newData);
fclose($handle);
}
?>

This is the main line I’m having an issue with: ” [B]$myfile = “bio.txt”;[/B] “

I would like to automatically add this “[B]bio.txt[/B]” file in the folder of the current logged in user.

For example, It works if I do it this way: ” [B]$myfile = “user/johndoe/bio.txt”;[/B] “
” [B]$myfile = “user/greg/bio.txt”;[/B] “
” [B]$myfile = “user/tammy/bio.txt”;[/B] “

What I want is something like this ” [B]$myfile = “user/$u/bio.txt”;[/B] or ” [B]$myfile = “user/$username/bio.txt”;[/B] ” but neither of those work. I need this one line to auto detect the user who is logged in so when that user updates his/her “Biography” section it will automatically edit and read their “bio.txt” which is in their user folder along with their pics, vids, mp3s, and other files.

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