Where is the php file, that does the checks for login information?

Question

I would like to add some additional checks into my wordpress website's username and password authentication function for specific usernames, but I cannot find the correct php file...

/* I would like to do something like this */
if ($username == "testuser_with_no_rights") {
    /* ... */
}
else { 
    /* do everything as it was defined originally */ 
}

So, basically, where is the php file, that does the checks for login information?

It is also possible, that there is a much better way of doing this. If that is the case, I am open for suggestions too.

Answer 1

You don't want to be messing around with that. Look at the following hook: https://codex.wordpress.org/Plugin_API/Action_Reference/wp_login

You should do something like:

function check_test_user() {
// Your code here
}

add_action('wp_login', 'check_test_user');

The hook will run as part of the Login process and saves you modifying core Wordpress files.