That helps a lot. The errors you are seeing are letting you know that the db variables on the page do not have the proper values. These variables are defined and set in the connections file. If this connections file is not included before the variables are referenced then you will get these errors. You need to ensure that you have the include for the connection file at the top of the page and that the connection file is properly crafted.
The connection file include line should be at the top and it looks like this by default:
<?php require_once('../Connections/localhost.php'); ?>
It is looking in the parent directory for the Connections folder and the file inside of it. Also, you must ensure that you have the proper values in this file.