如何清除$_SERVER['PHP_AUTH_USER'] ？？？？

Question

<?              //这是文件auth.inc              /*验证用户身份信息*/              if(!isset($PHP_AUTH_USER))                  {                  header("WWW-Authenticate:Basic realm=\"访问权限验证\"");                  header("HTTP/1.0 401 Unauthorized");                  echo "您没有输入用户名，您无权登录到本站!";   header("WWW-Authenticate:Basic realm=\"访问权限验证\"");                  header("HTTP/1.0 401 Unauthorized");                  exit();                  }              /*连接数据库*/              else                {                   include('config.php');                  /*查询用户是否存在*/                   $query1="select password,enable from adminaccess where username='$PHP_AUTH_USER' ";   $result=mysql_query($query1);                       if(!$result)                            {                                  header("WWW-Authenticate:Basic realm=\"身份验证功能\"");                               header("HTTP/1.0 401 Unauthorized");                                echo "身份验证失败，您没有在本站注册!";    header("WWW-Authenticate:Basic realm=\"身份验证功能\"");                               header("HTTP/1.0 401 Unauthorized");                                exit();                             }                      else                           {      $row=mysql_fetch_array($result);                             $password = $row[0];                              $enable = $row[1];                                                         if($enable==0)                                    {echo "你没有访问权限";     header("WWW-Authenticate:Basic realm=\"身份验证功能\"");                                    header("HTTP/1.0 401 Unauthorized");                                       exit();}                             else if ($password!=$PHP_AUTH_PW)                                    {                                                                              header("WWW-Authenticate:Basic  realm=\"身份验证功能\"");                                       header("HTTP/1.0 401 Unauthorized");                                        echo "身份验证失败，您无权登录到本站!";    header("WWW-Authenticate:Basic realm=\"身份验证功能\"");                                      header("HTTP/1.0 401 Unauthorized");                                       exit();                                      }                                         }                        }              ?>