i included the following code in one file.

<?php
 if(!isset($PHP_AUTH_USER))
  {
    header('WWW-Authenticate: Basic Realm="Enter your login name and password"');
header('HTTP/1.0 401 Unauthorized');
    echo 'Authorization required';    exit;
  }      
  else  {    require("mysqlconn.inc");    
    $sql = "select password from users where username = '$PHP_AUTH_USER'";
    $result = mysql_query($sql,$conn);    $row = mysql_fetch_array($result);
   if($PHP_AUTH_PW != $row[0])
    {     header('WWW-Authenticate: Basic Realm="Authorization failed : Enter Username and password"');
    header('HTTP/1.0 401 Unauthorized');
     echo 'Authorization required';
     mysql_free_result($result);
     mysql_close();
     exit;
   } 
    mysql_free_result($result);
    mysql_close();
  }
?>


I have included the following code to logout the user

<?
unset($PHP_AUTH_USER);
?>

but it's not destroying the variable $PHP_AUTH_USER.