auth_redirect()

function auth_redirect() {
	// Checks if a user is logged in, if not redirects them to the login page

	$secure = ( is_ssl() || force_ssl_admin() );

	/**
	 * Filters whether to use a secure authentication redirect.
	 *
	 * @since WP-3.1.0
	 *
	 * @param bool $secure Whether to use a secure authentication redirect. Default false.
	 */
	$secure = apply_filters( 'secure_auth_redirect', $secure );

	// If https is required and request is http, redirect
	if ( $secure && !is_ssl() && false !== strpos($_SERVER['REQUEST_URI'], 'wp-admin') ) {
		if ( 0 === strpos( $_SERVER['REQUEST_URI'], 'http' ) ) {
			wp_redirect( set_url_scheme( $_SERVER['REQUEST_URI'], 'https' ) );
			exit();
		} else {
			wp_redirect( 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
			exit();
		}
	}

	/**
	 * Filters the authentication redirect scheme.
	 *
	 * @since WP-2.9.0
	 *
	 * @param string $scheme Authentication redirect scheme. Default empty.
	 */
	$scheme = apply_filters( 'auth_redirect_scheme', '' );

	if ( $user_id = wp_validate_auth_cookie( '',  $scheme) ) {
		/**
		 * Fires before the authentication redirect.
		 *
		 * @since WP-2.8.0
		 *
		 * @param int $user_id User ID.
		 */
		do_action( 'auth_redirect', $user_id );

		// If the user wants ssl but the session is not ssl, redirect.
		if ( !$secure && get_user_option('use_ssl', $user_id) && false !== strpos($_SERVER['REQUEST_URI'], 'wp-admin') ) {
			if ( 0 === strpos( $_SERVER['REQUEST_URI'], 'http' ) ) {
				wp_redirect( set_url_scheme( $_SERVER['REQUEST_URI'], 'https' ) );
				exit();
			} else {
				wp_redirect( 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
				exit();
			}
		}

		return;  // The cookie is good so we're done
	}

	// The cookie is no good so force login
	nocache_headers();

	$redirect = ( strpos( $_SERVER['REQUEST_URI'], '/options.php' ) && wp_get_referer() ) ? wp_get_referer() : set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );

	$login_url = wp_login_url($redirect, true);

	wp_redirect($login_url);
	exit();
}