PHP Mysql Error: pdo_mysql extension is not installed.

Magento needs PDO Mysql extension for database connection and related things, so if you don’t have pdo_mysql extension enabled Magento will complain about this and will not proceed further installation. If you are not sure what PDO is, it’s high time for you to look at http://php.net/manual/en/ref.pdo-mysql.php

Coming back to error, to resolve this you will need to edit your php.ini file where it says:
;extension=pdo_mysql.so

Just uncomment the line by removing front semincolon, so it becomes
extension=pdo_mysql.so

Save it and restart the server, the error should go.

If you are on Windows, then that line should read:

extension=php_pdo_mysql.dll

If you don’t find pdo_mysql in php.ini, install php5-mysql by running the command:

sudo apt-get install php5-mysql (on Ubuntu)

sudo yum install php-mysql (on Redhat, Fedora, CentOS)