PHP: PHP4 AND MySQL Authentication

<?php
require ("auth.php");
if(!isset($PHP_AUTH_USER))  {
   Header("WWW-Authenticate: Basic realm=\"User Login\"");
   Header( "HTTP/1.0 401 Unauthorized");
    echo "You failed to provide the correct password....\n";
   exit;
}
   else {
   $con = mysql_pconnect ("$host", "$user", "$pass")  or die("Error: " . mysql_error());
   mysql_select_db ("$db");
   $user_id = strtolower($PHP_AUTH_USER);
   $result = mysql_query("SELECT password FROM signup " . "Where username = '$user_id'");
   $row = mysql_fetch_array($result);
if ($PHP_AUTH_PW != $row["password"])  {
   Header( "WWW-Authenticate: Basic realm=\"Login failed please try again!\"");
   Header( "HTTP/1.0 401 Unauthorized");
    echo "You failed to provide the correct password....\n";
   exit;
  }
}
?>