1. a 000webhost.com server
That is a curious new server provider. I might have to recommend this for beginners instead of AWS.
2. which is exactly the difference between normal and dedicated servers?
I would consider dedicated servers "normal" and peer-to-peer as an oddball. Your 000webhost server is a dedicated server.
See: how-do-i-use-a-lobby-server-for-a-multiplayer-game_p1091393?#p1091393
I tried using the AJAX object to "contact" a database, but my problem is that all the tutorials are written for PHP 5.6 (which uses MySQL) and since the PHP 7 update, it is necessary to write MySQLi (I don't know and I didn't find the adequate code), so I decided trying with the Multiplayer plugin (which would be fine if the room's stability didn't depend on the host being connected). I don't want to give up the Multiplayer plugin yet, I hope to be able to find a solution.
Anyhow, if you could kindly tell me what I exactly need to research on to implement a dedicated server (I do only have basic knowledge), it would be great.
Thank you once again for your help, I really need it <img src="{SMILIES_PATH}/icon_e_smile.gif" alt=":)" title="Smile">
MySQLi for beginners: http://codular.com/php-mysqli
For research to use a dedicated server, you just need to use your 000webhost server and do research on "PHP program application interface tutorial" and "PHP mySQLi" to find out how to take in HTTP parameters and place them into your database. Then you access your API via AJAX and use it to input items into your database.