PHP 5 OOPS concepts Key Words

Extends
In PHP a class a class can inherit methods, functions and members of other class by using the extends keyword in the declaration. In PHP it is not possible to inherit from multiple classes, a class can inherit from only one base class.

The class from which inheritance is done is called the parent class or base class and the class which inherits is called the child class.

Final
The final keyword prevents the child classes from overriding a method. This can be done by prefixing the method with the keyword final. If the complete class is being defined as final then that class cannot be extended.

Abstract
A new concept of abstract classes and methods has been introduced in PHP5. When a class is defined as abstract then it is not allowed to create the instance of that class. A class that contains at least one abstract method must also be abstract. The methods defined as abstract cannot define the implementation; they just declare the method’s signature.

When a child class is inheriting from an abstract parent class, then all the methods marked abstract in parent class declaration must also be additionally defined by the child class. These methods must be defined with the same or weaker access. This means that if an abstract method is declared as protected in the parent class then it must be declared either as protected or public in the child class.

Static
When class members or methods are declared as static then there is no need to instantiate that class. These members and methods are accessible without the instantiation of the class. If a member is declared as static then it cannot be accessed by an instantiated class object, but a method declared as static can be accessed by an instantiated class object.

The static declaration of a class must be after the visibility declaration (means that after the member or method has been declared as public, protected, or private).

The static method calls are resolved at compile time and static properties cannot be accessed through the object through the arrow operator (->).

Interfaces
Object interfaces allow the creation of a code which specifies that which method a class must implement, without having to define how these methods have to be handled.

Interfaces are defined in the same way as a class is defined. These interfaces are defined with the keyword “interface”. In the interface the contents of the methods do not have to be defined and all the methods declared in the interface must be declared as public.

Implementation of Interfaces
To implement an interface, the implements operator is used. The methods must be defined before implementation and all the methods in the interface must be implemented within a class.