Open browser password dialog and authenticate user based on database - PHP Script

<?php
   function authenticate_user() {
      header('WWW-Authenticate: Basic realm="Secret Stash"');
      header("HTTP/1.0 401 Unauthorized");
      exit;
   }

   if(! isset($_SERVER['PHP_AUTH_USER'])) {
      authenticate_user();
   } else {
      mysql_connect("localhost","authenticator","secret") or die("Can't connect to database server!");
      mysql_select_db("gilmorebook") or die("Can't select authentication database!");

      $query = "SELECT username, pswd FROM user WHERE username='$_SERVER[PHP_AUTH_USER]' 
                      AND pswd=MD5('$_SERVER[PHP_AUTH_PW]') AND ipAddress='$_SERVER[REMOTE_ADDR]'";

      $result = mysql_query($query);
      if (mysql_num_rows($result) == 0)
         authenticate_user();
         mysql_close();
      }
?>