You could change the link to the registration page:
<a href="<?php echo($assumedRoot); ?>users_Registration.php">Register</a>
to:
<a href="<?php echo($assumedRoot); ?>users_Registration.php?accesscheck=<?php echo ($_SERVER["PHP_SELF"]);?>">Register</a>
you will also need to change the forms tag on the registration page:
<form method="post" id="WAATKRegistrationForm" >
to:
<form method="post" id="WAATKRegistrationForm" action="<?php echo $_SERVER["PHP_SELF"]; ?><?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != "")?"?".$_SERVER["QUERY_STRING"]:""; ?>">