Source

File: wp-includes/wp-db.php

public function __construct( $dbuser, $dbpassword, $dbname, $dbhost ) {
		if ( WP_DEBUG && WP_DEBUG_DISPLAY ) {
			$this->show_errors();
		}

		// Use ext/mysqli if it exists unless WP_USE_EXT_MYSQL is defined as true.
		if ( function_exists( 'mysqli_connect' ) ) {
			$this->use_mysqli = true;

			if ( defined( 'WP_USE_EXT_MYSQL' ) ) {
				$this->use_mysqli = ! WP_USE_EXT_MYSQL;
			}
		}

		$this->dbuser     = $dbuser;
		$this->dbpassword = $dbpassword;
		$this->dbname     = $dbname;
		$this->dbhost     = $dbhost;

		// wp-config.php creation will manually connect when ready.
		if ( defined( 'WP_SETUP_CONFIG' ) ) {
			return;
		}

		$this->db_connect();
	}