PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

172 Chapter 6 Object-Oriented PHP
In the same way that providing new attributes or operations in a subclass does not affect
the superclass, overriding attributes or operations in a subclass does not affect the superclass.
A subclass will inherit all the attributes and operations of its superclass, unless you
provide replacements. If you provide a replacement definition, it takes precedence and
overrides the original definition.
The parent keyword allows you to call the original version of the operation in the
parent class. For example, to call A::operation from within class B, you would use
parent::operation();
The output produced is, however, different. Although you call the operation from the
parent class, PHP uses the attribute values from the current class. Hence, you get the following
output:
Something
The value of $attribute is different value
Inheritance can be many layers deep.You can declare a class imaginatively called C that
extends B and therefore inherits features from B and from B’s parent, A.The class C can
again choose which attributes and operations from its parents to override and replace.
Preventing Inheritance and Overriding with final
PHP uses the keyword final.When you use this keyword in front of a function declaration,
that function cannot be overridden in any subclasses. For example, you can add it
to class A in the previous example, as follows:
class A
{
public $attribute = "default value";
final function operation()
{
echo "Something";
echo "The value of \$attribute is ". $this->attribute."";
}
}
Using this approach prevents you from overriding operation() in class B. If you attempt
to do so, you will get the following error:
Fatal error: Cannot override final method A::operation()
You can also use the final keyword to prevent a class from being subclassed at all.To
prevent class A from being subclassed, you can add it as follows:
final class A
{...}
If you then try to inherit from A, you will get an error similar to
Fatal error: Class B may not inherit from final class (A)