How to create a platform or portable code to save your time?

I've created many websites using PHP,XHTML, CSS and MySQL, and everytime I want to create a new website I have to do the following:

1.creation and conception of the admin pages. 2.creation of the website from scratch

but the problem is that , these days we founded a new company for websites creation.. and I am the lonely programmer and webdesigner, we don't won't to use Joomla , or drupal.. and I want to create a platform that helps me save the time.

so this is my question:

How can I create a joomla like platform, with many options (style manager, articles manager, categories manager...etc)... ?
I've decided to create an windows application, but I don't know if it's a good idea, though I don't know from where to begin.

I am a good designer, but while programming, I don't know the rules to create portable code. so I redo all the work in every project.

thank you all.

Solution

The keyword is asset.

Everything you create for one customer should become an asset among many others that you own, that way each time you get a new inquiry, you leverage the assets you already have to avoid recreating many parts. When you'll have done a lot of projects you'll end up with a descent platform that saves you time and money.

I'm telling you this since you state you aren't an expert developer, because on another hand, you can simply do one big product (CMS,...) that you'll adapt to each customer you get but this is damn tedious and require sharp conceptual and programmatic knowledge.

Even if you wish to avoid it, you can still turn to the existing CMSs and master one of them that will become your work base.

My two cents