PHP_SELF is a pre-defined variable that you can define as:
$self = $_SERVER;
To make it work strictly with PHP, define a form with the action being to $self (after defining it as above). You can define a form with just the drop-down for the first pass and if it is set then display a different form based on what they chose. The screen will refresh and you will display the second form. That can have an action of $self as well but obviously you need to test the values that you receive to know what form they submitted. You may want to use a consistent hidden variable for that.
That's the theory. If you have a reasonable understanding of PHP and HTML you should be able to code that up in a flash. Another thing to note is that you need braces for the switch statement.
Chris