[Carpetsmoker]

You don't need to manually add extensions to /usr/local/etc/php.ini, the ports system automaticlly adds extensions to /usr/local/etc/php/extensions.ini, you just need to restart your webserver and it works (You did restart your webserver, right?)

I have uncommented the extension of mysqli in extension=php_mysqli.so.

I think you uncommented extension=php_mysql.dll, which is only for MS Windows systems.

You can check if the mysqli extension is loaded with this PHP code:

var_dump(extension_loaded('mysqli'));