User Authentication

Hi, Using the following code I am able to authenticate which type of user is visiting my page, however if I try to log in again with a different type of user the session variables still assume that the original user was logged in, is there a way to reset the session variables, I have tried session_destroy() and session_unset() but without success... <?php require("dbconnect.php"); // Assume user is not authenticated $auth = false; // Formulate the query $query = "SELECT * FROM WMS_User WHERE User_Username = '$_POST[username]' AND User_Password = '$_POST[password]'"; // Execute the query and put results in $result $result = mysql_query( $query ) or die ( 'Unable to execute query.' ); // Get number of rows in $result. $num = mysql_numrows( $result ); if ( $num != 0 ) { // A matching row was found - the user is authenticated. $auth = true; //get the data for the session variables $suser_name = mysql_result($result, 0, "User_Name"); $suser_password = mysql_result($result, 0, "User_Password"); $stype_level = mysql_result($result, 0, "User_Type"); $ses_name = $suser_name; $ses_pass = $suser_password; $ses_level = $stype_level; session_register("ses_name"); session_register("ses_pass"); session_register("ses_level"); } //if user isn't authenticated redirect to appropriate page if ( ! $auth ) { include("index.php"); exit; } //if user is authenticated, include the main menu else{ include("home.php"); } //close connection mysql_close(); ?> thanks for your help