PHP Authetication

Currently I am using PHP authetication using header function as below :-
Header("WWW-authenticate: basic realm=Restricted Area") ;
Header("HTTP/1.0 401 Unauthorized") ;

If the user has not been authenticated, the above code works fine. Sometimes, my user would like to login as another person. I set up a button to call the above code again. Although the login screen pop up again, it does not accept the new sign on.

How should I do the coding so that my user can sign on the second time with a different login id ?

Thank you for your help.