Write FORM data to text file?

I'd like to create a form with 3 fields, which will
take the contents of each of the 3 fields and store
them in a text file.

==============================================
Form
==============================================

Enter Website URL Here: [_________________]
Enter Account ID# Here: [_________________]
Enter Email Address Here: [_________________]

[ SUBMIT ]

==============================================



When the user submits that form, the information
gets written to a text file containing only this:

==============================================
Text file named info.inc
==============================================

<?php
$weburl = "WebSiteName.com";
$accountid = "12345";
$email = "usersemail@anymailaddress.com";
?>

==============================================


Any time the user goes back to the form above
and changes the data in any of the 3 fields,
the text file will be overwritten accordingly.

Note: The text file will have the CHMOD
permission set to 644 so nobody else can
write to it.

Note: The form will be in a password protected
folder.

No help needed to create the form and
password protected folder. I just need to
know how to get the form SUBMIT to overwrite
the text file to contain the new form data.

Thanks!

Gene