OT: How to use html form input to append input to a file?

[joe at actionline.com]

The form input model below works fine to send form input via email, but
how can I modify it to write (append) the form input to a text file in a
designated directory in my web hosted space instead of to email?

<form action=http://www.bluehost.com/bluemail method=POST>
Email address: <input size=35 maxlength=90 name=Emailfrom><br>
Enter message:<br> <textarea name=Message rows 7 cols=50
wrap=virtual></textarea>
<input type=hidden name=subject value=Subject>
<input type=hidden name=required value= message>
<input type=hidden name=sendtoemail value=feedback at URLaddress.com>
<input type=hidden name=redirect value=http://www.URLaddress.com>
<input type=submit value=Send>
</form>