On a typical non-shared el6 box, php will read additional extensions/config from /etc/php.d/ and find mysqli.ini there IF php-mysqli is installed and loaded.
If you are on a shared el6 system the provider may not have it setup that way. Providers of most shared server offerings do/should have an FAQ or KB for customers. Checked that?
Please see: http://us3.php.net/phpinfo There is an example .php file there.
Can create it locally with NotePad or some text editor (NOT Word - copy and paste from page linked above).
Save the file as 'phpinfo.php' and place it in /var/www/html/ (ie, apache document root).
Access directly with browser: http://yoursite/phpinfo.php
'spirit of sharing', Ken