Source

File: wp-includes/load.php

function require_wp_db() {
	global $wpdb;

	require_once ABSPATH . WPINC . '/wp-db.php';
	if ( file_exists( WP_CONTENT_DIR . '/db.php' ) ) {
		require_once WP_CONTENT_DIR . '/db.php';
	}

	if ( isset( $wpdb ) ) {
		return;
	}

	$dbuser     = defined( 'DB_USER' ) ? DB_USER : '';
	$dbpassword = defined( 'DB_PASSWORD' ) ? DB_PASSWORD : '';
	$dbname     = defined( 'DB_NAME' ) ? DB_NAME : '';
	$dbhost     = defined( 'DB_HOST' ) ? DB_HOST : '';

	$wpdb = new wpdb( $dbuser, $dbpassword, $dbname, $dbhost );
}