:: What is MySQLi
The MySQLi Extension (MySQL Improved) is a relational database driver used in the PHP programming language to provide an interface with MySQL databases.

:: How to compile MySQLi Extension in PHP

cd /usr/local/src/php-5.3.1/
./configure --with-mysqli=mysqlnd --with-ndo-mysql=mysqlnd --with-mysql ...another..options..
make && make install

:: Checking MySQLi in PHP modules

php -m | grep mysqli

:: Restarting httpd

apachectl stop
apachectl start

:: Links
+ Google