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