Dear davidjennings
Your code is a miss mash of different coding technics.
You are using the MySQLi connection method but as MySQL.
For example:
MySQL
$con = mysql_connect('host','user','password'); <-- Notice no database selected
http://php.net/manual/en/function.mysql-connect.php
Where as your line 2 of code snippet 2 shows this:
$conn = mysql_connect("localhost","user_david","password","djenning_databaseclass"); **<-- You cannot select a database during MySQL connect**
MySQLi
$con = mysqli_connect('host','user','password','database');
http://www.php.net/manual/en/mysqli.construct.php
Sp you must decide which you are going to be using. As you have stated you want to use OOP.
If this is the case you can use MySQLi, or as I have already mentioned PDO with MySQL.