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Before beginning, though, let's make sure that you have a clear idea of the concepts involved here.
In PHP, a class is simply a set of program statements which perform a specific task. A typical class definition contains both variables and functions, and serves as the template from which to spawn specific instances of that class.
These specific instances of a class are referred to as objects. Every object has certain characteristics, or properties, and certain pre-defined functions, or methods. These properties and methods of the object correspond directly with the variables and functions within the class definition.
Once a class has been defined, PHP allows you to spawn as many instances of the class as you like. Each of these instances is a completely independent object, with its own properties and methods, and can therefore be manipulated independently of other objects. This comes in handy in situations where you need to spawn more than one instance of an object - for example, two simultaneous database links for two simultaneous queries, or two shopping carts.
Classes also help you keep your code modular - you can define a class in a separate file, and include that file only in the scripts where you plan to use the class - and simplify code changes, since you only need to edit a single file to add new functionality to all your spawned objects.
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