Fist of all, your form doesn't need to be in PHP, simple HTML will work. Next, in the PHP that is receiving your form, what you need to do to access the form options is something like this.
This is your form on you .htm page.
<form method="POST" action="recieving_file.php">
<input type=text name=kasettnummer size=1>
<input type=submit value='Search'>
and you receiving_file.php
<?PHP
echo $kasettnummer;
?>
That will print the value of kasettnummer to the screen. hth...