Page 246 - Beginning PHP 5.3

Part II: Learning the Language
H ow I t W orks
This script creates an interface, Sellable , that contains three method declarations:

public function addStock( $numItems );
public function sellItem();
public function getStockLevel();
Next, two classes — Television and TennisBall — are created. These classes are unrelated and

contain quite different properties and methods; for example, Television contains a private
$_screenSize property and methods to access it, whereas TennisBall contains a private $_color
property with associated methods.
However, both classes implement the Sellable interface. This means that they must provide the code

to implement the three methods — addStock() , sellItem() , and getStockLevel() — declared in
Sellable . This they do. Notice, by the way, that each class has a different way of recording its stock;
Television records the stock level in a $_stockLevel property, whereas TennisBall has a
$_ballsLeft property. This doesn ’ t matter at all; from the perspective of the outside world, the
important thing is that the classes correctly implement the three methods in the Sellable interface.

Next, the script creates a StoreManager class to store and handle products for sale in the online store.
This class contains a private $_productList array to store different types of products; an
addProduct() method to add product objects to the product list; and a stockUp() method that
iterates through the product list, adding 100 to the stock level of each product type.

stockUp() calls the addStock() method of each object to add the stock; it knows that such a method
must exist because the objects it deals with implement the Sellable interface. Notice that
addProduct() uses type hinting to ensure that all objects that it is passed implement the Sellable
interface (you can use type hinting with interface names as well as class names):
public function addProduct( Sellable $product ) {

Finally, the script tests the interface and classes. It creates a new Television object, $tv , and sets its
screen size to 42 inches. Similarly, it creates a TennisBall object, $ball , and sets its color to yellow.
Then the script creates a new StoreManager object, $manager , and adds both the $tv and $ball
product types to the stock list using the addProduct() method. Once the products are added,
$manager - > stockUp() is called to fill the warehouse with 100 units of each item. It then displays

information about each product, calling functions specific to the Television and TennisBall classes
( getScreenSize() and getColor() , respectively) as well as the getStockLevel() function declared
by the Sellable interface.

The script then sells some stock by calling the sellItem() method of both the $tv and $ball objects —
again, remember that this method is required by the Sellable interface — and redisplays information
about both products, including their new stock levels.

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