how to enable MySQLi extension for PHP in htaccess with example please thank you vaery much
You don't.
Mysqli should be installed as a PHP-module and loaded through php.ini.
Assuming you use a debian-related Linux distribution, this can be done by issuing apt-get install php5-mysqli.
If this is not an option, please read the PHP documentation on this module and follow the steps described there.
The extension= directive is only valid in the central php.ini, it cannot be set from within .htaccess files.
At best you could load it manually in your PHP script:
<?php
dl("mysqli.so");
Only works until PHP 5.2