If you are familiar with php, a good practice is to use a php framework to build your applications. One choice is CodeIgniter.
CodeIgniter is based on the Model-View-Controller development pattern. MVC is a software approach that separates application logic from presentation. In practice, it permits your web pages to contain minimal scripting since the presentation is separate from the PHP scripting.
The Model represents your data structures. Typically your model classes will contain functions that help you retrieve, insert, and update information in your database.
The View is the information that is being presented to a user. A View will normally be a web page, but in CodeIgniter, a view can also be a page fragment like a header or footer. It can also be an RSS page, or any other type of “page”.
The Controller serves as an intermediary between the Model, the View, and any other resources needed to process the HTTP request and generate a web page.
CodeIgniter also enables you to incorporate your own existing scripts, or even develop core libraries for the system, enabling you to work in a way that makes the most sense to you. (source: http://ellislab.com/codeigniter/user-guide)