Confirm that mysqli extension is not installed

You can confirm that mysqli is installed, or not, by listing the installed modules. SSH into your Cent OS box, and issue the following command.

php -m | grep mysqli

If nothing is returned, then you do not have mysqli.so loaded. Check if you have the shared object is installed on you system.

# Located extension dir
php -i | grep extension_dir
# List mysql.so in the path returned from the previous command
ls -la /usr/lib/php/extensions/no-debug-non-zts-20090626/mysqli.so

If the mysqli.so is present, and has the permissions -rwxr-x-rx, you'll need to load/enable the mysqli extension in the systems global php.ini file.

# Adjust path to correct php.ini file. 
# Run `php -i | grep "Configuration File"` to locate, if needed
# It might be easier to use vi, or nano, for this
sudo echo "extension=mysqli.so" >> /etc/php5/php.ini
# Restart apache
sudo service httpd restart

Else. If you do not have mysqli.so present in your system. You can install the rpm by following your systems package manager, and repeating the previous php.ini step.

sudo yum install php5-mysqli