Honestly, If you have to post questions all across the internet to learn how to secure your system, then a BitCoin gambling site may not be for you.
If I was programming a bitcoin gambling site I wouldn't share any of my sites inner workings anywhere online.
I would suggest asking specific questions such as, What is the benefits of writing to my database using PDO vs mysqli or mysql?
The other thing you could do is hire a professional to come in and secure your system for you.
Good luck.
Thanks for the comments - I am asking at a high level as I want to make sure that this is as secure as possible.
The advice of hiring a professional is certainly something I am considering but this in itself poses risks!
Posting the questions isn't for learning - but deciding on the best approach. I am not new to computer security but this is my first Bitcoin related project.