As for your first query, to soften the load on servers, use count() instead.
$mysqli->query("SELECT count(*) FROM RegisteredUsersTable WHERE UserEmail = '$newUserEmail'");
This way, you can just check if you've gotten a result higher than 1. If the result is greater than 1, then the username exists (Since a row was returned).
To check the data returned, you need to simply execute the statement, then fetch the results. Part of the fun is learning, so here's the documentation
