So, you're working on a project you're unwilling to share code of... very recently, you asked for opinions on billing software. It's a fairly easy connection to see what you're working on - but to be brutally honest, the best thing you could do at this point is don't.
This isn't trying to rip your effort, or make fun of you; for your own good, take a step back and look at the situation objectively. You're a novice with php, a language which already has fundamental flaws that have to be taken into consideration when dealing with security. It's a reasonable guess that you're wanting to build a billing panel, or at least an operational replacement for WHMCS (perhaps even to monitize). But you HAVE to know your limits.
Look at your recent KVM issue - you opened, closed, and reopened a brand all within 48 hours, due to inexperience, frustration, and lack of conviction. You're not ready to undertake projects on your own - especially given you only have a passing knowledge of the language in question. And that's not even touching on just how complex a billing system can get. That's why we're seeing so many quickly-abandoned panel projects; people think it's easy to just throw some code down, and very soon get frustrated with how much actual work is involved and give up. Or in the case of DimeCadium or a certain 'developer' we used to work with, just taking someone's money and running without having any intent of producing.
Seriously, save yourself headache and embarrassment now - don't try to do anything more complex than a single-user blog-type website by yourself. You have neither the experience nor the coding skill for something that large. Get involved with some simple open-source projects - really get a good grasp of the languages and tie-ins (such as the finer points of PROPER mySQL - you can't just play with mysql_query() and think you know how DBs really work). Spend a couple of years building a solid understanding of the tools you'll need. After all - you're going to be asking your clients to trust your work to keep their information secure.
Last, but probably most important - before you do ANY type of attempt at a panel, learn how existing ones work first. Don't just skim documentation - actually spend some time writing modules for systems like WHMCS. Teach yourself to do the simple point-and-click actions using the more complex API, or even writing short scripts to do the work. Yes, this is 'excessive work', but it helps you learn just how complex the panel really is, and just how important all the various pieces are to each other.
A food-for-though example: in your WHMCS database, there are over 90 tables. Two of them are tblproductconfigoptionssub and tblhostingconfigoptions, for example. Do you know what those two tables do? Do you know how they interact with multiple other tables? I will be the first to tell you that WHMCS is very, very poorly written. But the bottom line is - they've build a functional billing panel, and you haven't. Learn from them. Use an existing setup to familiarize yourself with what you will need to do.
But again, first and foremost. Learn the basics. Temper your impatience, and try to realize now that every little mistake you make is going to seriously affect your credibility down the line. Not because you made a mistake, everyone does that. But because you dove headfirst into something you're not ready for - a VERY scary idea once people realize they're trusting you with their data.
