Object-oriented programming

We assume that you have basic knowledge of OOP. You may have learned it as an afterthought to "normal" PHP 4. PHP 4 is not an OO language, though some OO functionality has been stacked on to it. PHP 5 is much better, with an underlying engine that was written from the ground up with OO in mind.

You can do most of the basics in PHP 4, and CI manages to do everything it needs internally in either language. The key thing to remember—when an OO program is running, there is always one current object (but only one). Objects may call each other or hand over control to each other, in which case the current object changes, but only one of them can be current at any time. The current object defines the scope, in other words, the variables (properties) and methods (functions) that are available to the program at that moment. So it's important to know and control the current object.

PHP, being a mixture of functional and OO programming, also offers the possibility where...