Next, we've called class methods on the $objEmployee object to print the information which was initialized during object creation. Of course, you can create multiple objects of the same class, as shown in the following snippet. The following image is a graphical representation of the Employee class and some of its instances. Simply put, a class is a blueprint which you can use to create structured objects. ## Encapsulation In the previous section, we discussed how to instantiate objects of the Employee class. It's interesting to note that the $objEmployee object itself wraps together properties and methods of the class. In other words, it hides those details from the rest of the program. In the world of OOP, this is called data encapsulation.

Encapsulation is an important aspect of OOP that allows you to restrict access to certain properties or methods of the object. And that brings us to another topic for discussionaccess levels.

### Access Levels

When you define a property or a method in a class, you can declare it to have one of these three access levelspublic, private, or protected.

#### Public Access

When you declare a property or a method as public, it can be accessed from anywhere outside the class. The value of a public property can be modified from anywhere in your code.

Let's look at an example to understand the public access level.

As you can see in the above example, we've declared the name property to be public. Hence, you can set it from anywhere outside the class, as we've done here.

#### Private Access

When you declare a property or a method as private, it can only be accessed from within the class. This means that you need to define getter and setter methods to get and set the value of that property.

Again, let's revise the previous example to understand the private access level.

If you try accessing a private property from outside the class, it'll throw the fatal error Cannot access private property Person::$name. Thus, you need to set the value of the private property using the setter method, as we did using the setName method. There are good reasons why you might want to make a property private. For example, perhaps some action should be taken (updating a database, say, or re-rendering a template) if that property changes. In that case, you can define a setter method and handle any special logic when the property is changed. #### Protected Access Finally, when you declare a property or a method as protected, it can be accessed by the same class that has defined it and classes that inherit the class in question. We'll discuss inheritance in the very next section, so we'll get back to the protected access level a bit later. ## Inheritance Inheritance is an important aspect of the object-oriented programming paradigm which allows you to inherit properties and methods of other classes by extending them. The class which is being inherited is called the parent class, and the class which inherits the other class is called the child class. When you instantiate an object of the child class, it inherits the properties and methods of the parent class as well. Let's have a look at the following screenshot to understand the concept of inheritance. In the above example, the Person class is the parent class, and the Employee class extends or inherits the Person class and so is called a child class. Let's try to go through a real-world example to understand how it works. The important thing to note here is that the Employee class has used the extends keyword to inherit the Person class. Now, the Employee class can access all properties and methods of the Person class that are declared as public or protected. (It can't access members that are declared as private.) In the above example, the $employee object can access getName and setName methods that are defined in the Person class since they are declared as public.

Next, we've accessed the callToProtectedNameAndAge method using the getNameAndAge method defined in the Employee class, since it's declared as protected. Finally, the $employee object can't access the callToPrivateNameAndAge method of the Person class since it's declared as private. On the other hand, you can use the $employee object to set the age property of the Person class, as we did in the setAge method which is defined in the Employee class, since the age property is declared as protected.

So that was a brief introduction to inheritance. It helps you to reduce code duplication, and thus encourages code reusability.

## Polymorphism

Polymorphism is another important concept in the world of object-oriented programming which refers to the ability to process objects differently based on their data types.

For example, in the context of inheritance, if the child class wants to change the behavior of the parent class method, it can override that method. This is called method overriding. Let's quickly go through a real-world example to understand the concept of method overriding.

As you can see, we've changed the behavior of the formatMessage method by overriding it in the BoldMessage class. The important thing is that a message is formatted differently based on the object type, whether it's an instance of the parent class or the child class.

(Some object-oriented languages also have a kind of method overloading that lets you define multiple class methods with the same name but a different number of arguments. This isn't directly supported in PHP, but there are a couple of workarounds to achieve similar functionality.)

## Conclusion

Object-oriented programming is a vast subject, and we've only scratched the surface of its complexity. I do hope that this tutorial helped you get you started with the basics of OOP and that it motivates you to go on and learn more advanced OOP topics.

Object-oriented programming is an important aspect in application development, irrespective of the technology you're working with. Today, in the context of PHP, we discussed a couple of basic concepts of OOP, and we also took the opportunity to introduce a few real-world examples.

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