Inheritance

4. What is Inheritance (Is-a Relationship)?

• The mechanism of deriving a new class from existing class is called inheritance.
• The inheritance allows subclasses to inherit all properties (variables and methods) of their parent class.
• When a “Is-A” relationship exists between two classes we use Inheritance.
• Reusability is achieved by INHERITANCE
• PHP classes can be reused by extending a class. Extending an existing class is nothing but reusing properties of the existing classes.
• The class whose properties are extended is known as super or base or parent class.
• The class which extends the properties of super class is known as sub or derived or child class.
• The keyword extends is used by the sub class to inherit the features of super class.

4.1. Is-a Relationship

• When one object is a specialized version of another object, there is an “is a” relationship between them. For example, a grasshopper is an insect. Here are a few other examples of the “is a” relationship:
  1.  A poodle is a dog.
  2.  A car is a vehicle.
  3.  A flower is a plant.
  4.  A rectangle is a shape.
  5.  A football player is an athlete.
• When an “is a” relationship exists between objects, it means that the specialized object has all of the characteristics of the general object, plus additional characteristics that make it special.
• In object-oriented programming, inheritance is used to create an “is a” relationship among classes. This allows you to extend the capabilities of a class by creating another class that is a specialized version of it.

4.2. Basic Types of inheritance:

1. Single inheritance
2. Multilevel inheritance
3. Multiple inheritance
4. Hierarchical inheritance
5. Single inheritance