Is it possible to change submit button's URL link based on user input?

I am having a difficult time getting Flash to communicate with the php file that currently exists (pasted below for reference)  Ideally my Flash project would utilize that php file, or a slight variation thereof.  If I could get help making that happen, I would be grateful, but I am just trying to find a way around doing that because I can't figure that out.  Here is my flash project: http://www.jmusicphoto.com/fullscreen8_contact.php  If you click on the Client Access button, you can see the start of my form.  I simply want to instruct the user to enter their specific gallery code, then have the submit button link them into their gallery based on the code they input.

the php file:

<?php

/*
First off, we check to see if their is a submission for 'key'.
The form, which will call itself, will populate this when it is submitted.
If the form wasn't submitted this whole block will be ignored and the form will show.

It is important that the code go first because if we use php header redirection nary
a character can be output before the redirection takes place.
*/
if($_REQUEST['key']) {

     //now that we know it exists, deposit the key into a variable that is easier to type
     $key = $_REQUEST['key'];


     //switch is case sensitive, Banana, banana and banAna would all be different cases
     //strtolower() is a handy function that will take our string and force lower
     //case on every character.  If you want case-sensitivity leave this out
     $key = strtolower($key);


     
     //now that we have key, we need to compare it to all valid values, the switch statement
     //will make this fairly easy
     switch($key) {
          //add a case statement for every possible correct entry
          case 'passwordA':
               header('Location: http://www. randomsite. com/pageY.html');          
               break;
          case 'passwordB':
               header('Location: http://www. randomsite. com/pageY.html');          
               break;
          
          default:
               echo "Oh noes!  You got something wrong.  You typed " . $key . " maybe there is a mistake?<br><br>";
               break;
     }
}
?>

<!--
The form will call itself, so the action needs to be the same filename as this file.
It will pass along the key for our above function to check
-->
<form name='theform' action='thisfile.php'>
     <input type="text" name="key" />
    <input type="image" name="submit" src="submit.gif" border='0' style='vertical-align:top' width="100" height="20">
</form>