I have taken the following lines from DB_Connect.php and put them into the method 'getUserByIdAndPassword' in DB_Functions.php.
$con = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD); mysqli_select_db($con, DB_DATABASE) or die(mysqli_connect_error());
This then allows me to fix the syntax of msqli_query as so;
$result = mysqli_query($con, "SELECT * FROM smiths WHERE id = '$id'") or die(mysqli_error())
And this kids is why you don't use deprecated code like some Frankenstein madman.