Try something like this ...
<?php
if ($update == "yes") {
$text=stripslashes($_POST["text"]);
$filename="text.txt";
$file = fopen($filename, "w");
fputs($file, $text);
fclose($file);
}
?>
<HTML>
<HEAD>
<TITLE>title</TITLE>
</HEAD>
<BODY>
<FORM METHOD="post" ACTION="<?=$_SERVER_['PHP_SELF']; ?>">
<INPUT TYPE=hidden NAME="update" VALUE="yes">
<TEXTAREA NAME="text" COLS="20" ROWS="12">
<?
include 'text.txt';
?>
</TEXTAREA>
<P>
<INPUT TYPE=submit VALUE="Update Text">
</FORM>
</BODY>
</HTML>
Create a text file called text.txt and make sure your user can write to it. Then include the text file in any page you want.
You may want to do some error checking and validation on the submitted data.