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 check the MySQLi extension is enabled or not on server?
You can check the MySQLi extension from command line itself by executing the following command.
# php -m | grep mysql
how to enable MySQLi extension on web-server with cPanel?
We can simply enable the MySQLi extension by rebuilding the web-server with easyapache script. It is very simple, just run the below pasted scripts from the command line as root user, and select the MySQLi from the extension list. See the attached image for more details:
# /scripts/easyapache 
MySQLi
how to enable MySQLi extension on web-server without cPanel?
Compile the mysql with the MySQLi extension.
# ./configure --with-mysql=/usr --with-mysqli=/usr/bin/mysql_config
How to check the MySQLi extension is enabled on server?
# php -m|grep mysql
mysql
mysqli
That’s it :-)