Page 240 - Beginning PHP 5.3

Part II: Learning the Language
This is where abstract classes and methods come into play. By making a parent class abstract, you lay
down the rules as to what methods its child classes must contain. When you declare an abstract method
in the parent class, you don ’ t actually insert any code in the method; instead, you leave that up to the
child classes. You ’ re specifying what the child class must do, not how to do it.

To declare an abstract method, simply use the abstract keyword, as follows:

abstract public function myMethod( $param1, $param2 );
As you can see, you can also optionally specify any parameters that the method must contain. However,
you don ’ t include any code that implements the method, nor do you specify what type of value the
method must return.

If you declare one or more methods of a class to be abstract, you must also declare the whole class to be
abstract, too:

abstract class MyClass {
abstract public function myMethod( $param1, $param2 );

}
You can ’ t instantiate an abstract class — that is, create an object from it — directly:

// Generates an error: “Cannot instantiate abstract class MyClass”
$myObj = new MyClass;

So when you create an abstract class, you are essentially creating a template, rather than a fully fledged
class. You are saying that any child classes must implement any abstract methods in the abstract class
(unless those child classes are themselves declared to be abstract).

By the way, you can mix abstract and non - abstract methods within an abstract class. So your abstract
class might define behavior that is common to all possible child classes, while leaving the remainder of
the methods abstract for the child classes to implement.

The opposite of an abstract class — that is, a class that implements all the methods of its parent abstract
class — is called a concrete class
.
Now return to the Shape example. By creating the Shape class as an abstract class, you can add a
declaration for the abstract getArea() method, ensuring that all child classes of Shape have to
implement getArea() :
abstract class Shape {
private $_color = “black”;
private $_filled = false;

public function getColor() {
return $this- > _color;
}

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