Explain code

Hello. I have a PHP project using MySQL and ADODB.

define('ADODB_BASEFILE', APP_BASEDIR . '/lib/adodb518/adodb.inc.php');
define('DB_HOST', 'localhost');
define('DB_USER', 'root');
define('DB_KEY' , 'key');
define('DB_DATA', 'database');
define('DB_TYPE', 'mysqli');

PHP:

My ADODB class connection is:

<?php
require_once ADODB_BASEFILE;
class ConexionDB […]

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