wp_authenticate( string $username, string $password )

function wp_authenticate($username, $password) {
	$username = sanitize_user($username);
	$password = trim($password);

	/**
	 * Filters whether a set of user login credentials are valid.
	 *
	 * A WP_User object is returned if the credentials authenticate a user.
	 * WP_Error or null otherwise.
	 *
	 * @since WP-2.8.0
	 * @since WP-4.5.0 `$username` now accepts an email address.
	 *
	 * @param null|WP_User|WP_Error $user     WP_User if the user is authenticated.
	 *                                        WP_Error or null otherwise.
	 * @param string                $username Username or email address.
	 * @param string                $password User password
	 */
	$user = apply_filters( 'authenticate', null, $username, $password );

	if ( $user == null ) {
		// TODO what should the error message be? (Or would these even happen?)
		// Only needed if all authentication handlers fail to return anything.
		$user = new WP_Error( 'authentication_failed', __( '<strong>ERROR</strong>: Invalid username, email address or incorrect password.' ) );
	}

	$ignore_codes = array('empty_username', 'empty_password');

	if (is_wp_error($user) && !in_array($user->get_error_code(), $ignore_codes) ) {
		/**
		 * Fires after a user login has failed.
		 *
		 * @since WP-2.5.0
		 * @since WP-4.5.0 The value of `$username` can now be an email address.
		 *
		 * @param string $username Username or email address.
		 */
		do_action( 'wp_login_failed', $username );
	}

	return $user;
}