[How bout using your brain]

Ehm mysql root password is just stored in the mysql.user database. As a hash. So good luck with that.

Only thing you can do is to reset it, but you will probably have to have root (or mysql user) privs. which means you can just copy the whole mysql database anyway and resetting the root password would be then useless in that case.

[How bout using your brain]

I wouldn't be so sure about this 🙃

I am 100% certain about this 😝

I have found where I can get it, I just need to access to the directory, because PHP can't access to it, so I cannot download the file.

If you had access to that directory, you could just download the whole databases, no need to get root password...

[How bout using your brain]

Through a PHP file that I can edit, i can access to the entire server, but (I checked it on a Linux pc of mine) the folder of mysql is protected, and to unlock it I needed to use "chmod 0777" on linux terminal...

You can not. You have to be root or the owner of the file/directory. In your case that is the "mysql" user, while php is running under "www-data" user. You can not change the permissions of files you do not own.