I've found the problem hope this helps someone else.
Apparently tideSDK treats GET and POST in the same manner. So you don't have to specify the form method attribute on the forms and the form variables can be accessed using $_GET as shown in the code below.
<form action="login.php">
<input type="text" name="username" />
<input type="submit" name="submit" value="Submit" />
</form>
These can be retrived in login.php as:
<?php
print $_GET['username'];
?>