if you can access the .htaccess file you can add authentication. Also a php generated page could provide access using server variables for user/pass.
PHP can also store data to files & databases.
All depends on your server features and your coding ability ( HTML and/or php and / or javascript ).
I would have one php script that the arduino sends data to and the script simply appends a text file.
And you/users have a different php script to display the file contents.