MySQL Configuration on Windows Box with Apache and PHP

I am new to PHP and as well as with MySQL,

 I have installed PHP 5.28, on win XP (with Apache 2.09) and it is 
installed   properly (at least PHP and apache), 

as when I call phpinfo (), through a script, it shows

 the PHP Information.

then I installed MySQL 5.1.30, 
and from MySQL command prompt I   am  able to creating tables, making 
queries etc.

so i think it is also installed properly.

Now I want to use MySQL with PHP, but seems that *MySqli* is not  
installed properly. as when I write a following script in a php page I 
get a  error,

that indicates that i have written the following script in a file say 
*testmysql.php*.

*<?php
         $si = function_exists ( 'mysqli_connect' ) ;
         if ( $si )
         {
             echo ' MySQLi appears to be installed correctly ' ;
         }
         else
         {
             echo 'MySQLi <B> does not </B> appears to be installed. ' ;
         }
?>

I understand that it is a configuration error somewhere, as I am new 
with PHP and MySQL, so i  am feeling lost.

I have uncommented following two lines in the php.ini

*extension=php_mysql.dll
extension=php_mysqli.dll
*

and as well  these two lines are also uncommented in the httpd.conf 
(Apache config file ).

*LoadModule php5_module d:/programs/php528/php5apache2.dll
AddType application/x-httpd-php .php

*and theseb two files are also copied in windows folder (means available 
with windows path).*

php5ts.dll
php5apache2.dll

*
any guess what i am doing wrong, i am sure that it is some configuration 
error.

Prateek