Attempting to install CiviCRM 4.6.38 in Wordpress 4.9.8, PHP 7.2.5, MariaDB 10.3.8, Nginx 1.14.0, Amazon Linux 2. Getting "MySQL support not included in PHP". PHPinfo shows mysqli and mysqlnd enabled.
CiviCRM 4.6 is not compatible with PHP7.2 - you'll either need to upgrade CiviCRM, or downgrade PHP. (CiviCRM 4.6 is quite an old release to install in 2018.)
If the server's PHP does not have the appropriate function (
mysqli_connect() in recent CiviCRM, compatible with your PHP7.2, and
mysql_connect() in CiviCRM 4.6 - suitable for older PHP releases only as it was deprecated in PHP5.5 and removed in PHP7.0), then the installer will bail out saying that you "lack MySQL support in your PHP".
If you want to use CiviCRM 4.6, you'll need to run it with an older version of PHP.
I recommend using a current and supported version of both CiviCRM and PHP instead.