Object Orientation

How kind ... we've saved the most challenging for last! Let us close out this welcome (or should we say "wear out"!?) to PHP with a look at objects and the object-oriented programming in PHP. Objects are powerful but sometimes complex structures that have gained great popularity in programming. Before we look at PHP's implementation of objects, let's take a moment to ponder --- what is an object?

An object, in one metaphor, is a container. What does it contain? An object contains variables and it contains functions. That's pretty much it. Like soccer (aka "football"), it sounds a lot simpler than it is!

What might be an object? We could say that the concept of a "car" is an object. A car can possess a number of variables -- a list of colors, model name, model year, manufacturer, wheel size, fuel capacity, invoice price, and so on. We could even imagine that this "car" object has some functions -- a function that determines dealer price based on invoice price and certain features, for instance; or a function that determines fuel efficiency based on the variables of engine type and vehicle weight. The point is, all of this "stuff" is part of the "car" object.

In PHP, as in other object-oriented programming languages, you begin by creating a class, which is like a blueprint of an object. Rather than create a specific car object, for example, we create the outline of what a car object is and can contain.

class Car {
	var $colors;
	var $modelYear;
	var $modelName;
	var $modelMake;
	var $invoice;
	var $options;

	function dealerPrice ($carObject) {
	#add the values of each option to the invoice price
			$subTotal=0;
			while ( list($option,$cost)=each ($carObject->options) ) {
						$subTotal += $cost;
			}
			return $carObject->invoice + $subTotal;
	}
}

Voila -- we've just created the blueprint, or class, for a car object. The object has four scalar variables and two arrays ($colors and $options), as well as one function. The function simply sums up any values in the $options array and adds the result to the invoice price, resulting in what we're calling the "dealer price" (a pure fiction!).

Blueprint in hand, we can build an actual car object, or what is known as an instance. Suppose we want to create an instance of a new Porsche Boxter:

$boxter = new Car;

We've instantiated the car class, and now own a new but empty $boxter object. To access variables within the object, use the -> constructor:

$boxter->modelYear=1999;
$boxter->modelName="Boxter";
$boxter->modelMake="Porsche";
$boxter->invoice="85000";
$boxter->colors=array ("exterior"=>"red","interior"=>"black");
$boxter->options=array ("turbo"=>5000,"anti-theft"=>2500);

Assuming all values were in place, we could access the boxter object's function to calculate dealer cost:

if ($boxter->dealerPrice($boxter) > 90000) {
	print "Sorry, you can't afford a 
		 $boxter->modelYear $boxter->modelMake $boxter->modelName";
}

The Function of Functions
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