What can I do if I can write any php code to someone else's server and execute it?

Question

PHP

This code writes any file to the server. I can easily do it.
What is the best way to take advantage of the situation?
Connect to mysql database for example, if it is without a password or admin admin.
I don’t know Php and its features, and I rarely use sql databases. That's why I'm asking.
Interested in all the possible actions that I can take.

$name = uniqid();
  $name .= ".php";
  file_put_contents($name, base64_decode($_POST['data']));
  
  echo json_encode(array("src" => $name));

PS. This is a pure experiment, and my friend's server.

Answer 1

It all depends on what the task is.
It's elementary, if you can execute the script, and if you have access to the database, you can get all the necessary data from the same database.
From simply accessing a server on which there is no valuable information - there is no point.
Or, if a site with good traffic is spinning on this server, then you can set hidden links to your resources, etc.