PHP: Real world OOP example

OOP is not only about how a single class looks and operates. It's also about how instances of one or more classes work together.

That's why you see so many examples based on "Cars" and "People" because they actually do a really good job of illustrating this principle.

In my opinion, the most important lessons in OOP are encapsulation and polymorphism.

Encapsulation: Coupling data and the logic which acts on that data together in a concise, logical manner Polymorphism: The ability for one object to look-like and/or behave-like another.

A good real-world example of this would be something like a directory iterator. Where is this directory? Maybe it's a local folder, maybe it's remote like an FTP server. Who knows!

Here's a basic class tree that demonstrates encapsulation:

<?php

interface DirectoryIteratorInterface
{
    /**
     * @return \Traversable|array
     */
    public function listDirs();
}

abstract class AbstractDirectoryIterator implements DirectoryIteratorInterface
{
    protected $root;

    public function __construct($root)
    {
        $this->root = $root;
    }
}

class LocalDirectoryIterator extends AbstractDirectoryIterator
{
    public function listDirs()
    {
        // logic to get the current directory nodes and return them
    }
}

class FtpDirectoryIterator extends AbstractDirectoryIterator
{
    public function listDirs()
    {
        // logic to get the current directory nodes and return them
    }
}

Each class/object is responsible for its own method of retrieving a directory listing. The data (variables) are coupled to the logic (class functions i.e, methods) that use the data.

But the story is not over - remember how I said OOP is about how instances of classes work together, and not any single class or object?

Ok, so let's do something with this data - print it to the screen? Sure. But how? HTML? Plain-text? RSS? Let's address that.

<?php

interface DirectoryRendererInterface
{
    public function render();
}

abstract class AbstractDirectoryRenderer implements DirectoryRendererInterface
{
    protected $iterator;

    public function __construct(DirectoryIteratorInterface $iterator)
    {
        $this->iterator = $iterator;
    }

    public function render()
    {
        $dirs = $this->iterator->listDirs();
        foreach ($dirs as $dir) {
            $this->renderDirectory($dir);
        }
    }

    abstract protected function renderDirectory($directory);
}

class PlainTextDirectoryRenderer extends AbstractDirectoryRenderer
{
    protected function renderDirectory($directory)
    {
        echo $directory, "\n";
    }
}

class HtmlDirectoryRenderer extends AbstractDirectoryRenderer
{
    protected function renderDirectory($directory)
    {
        echo $directory, "<br>";
    }
}

Ok, now we have a couple class trees for traversing and rendering directory lists. How do we use them?

// Print a remote directory as HTML
$data = new HtmlDirectoryRenderer(
  new FtpDirectoryIterator('ftp://example.com/path')
);
$data->render();

// Print a local directory a plain text
$data = new PlainTextDirectoryRenderer(
  new LocalDirectoryIterator('/home/pbailey')
);
$data->render();

Now, I know what you're thinking, "But Peter, I don't need these big class trees to do this!" but if you think that then you're missing the point, much like I suspect you have been with the "Car" and "People" examples. Don't focus on the minutiae of the example - instead try to understand what's happening here.

We've created two class trees where one (*DirectoryRenderer) uses the other (*DirectoryIterator) in an expected way - this is often referred to as a contract. An instance of *DirectoryRenderer doesn't care which type of instance of *DirectoryIterator it receives, nor do instances of *DirectoryIterator care about how they're being rendered.

Why? Because we've designed them that way. They just plug into each other and work. This is OOP in action.

Whilst I know that this question has been answered already, I feel as though I can add value here.

I don't believe that you should use PHP as a programming language to learn OOP. If you wish to learn OOP for web applications, you should really be looking at C# or Java. Once you have learned OOP, then you can apply this knowledge to PHP. One example of a book I used to learn OOP was Big Java by Cay S. Horstmann

Why do I say this??? Because there are literally millions of examples on PHP of how to do stuff, however not many are examples of how to program properly. Further to this, PHP allows you to take many shortcuts, which would not be acceptable with the likes of Java. As such, if you program PHP with a Java head, then I believe that you will be a stronger programmer. OOP is not language specific, it is a programming paradigm.

If you must learn OOP using PHP, then I would recommend that you take a look at some real source code in public repositories of github. You can search them in packagist.org. If they are a decent public repository, they will contain a readme.md file which would show you how to use the composer packages. e.g https://github.com/moltin/laravel-cart is an example of a shopping cart package which you would be able to use in your application. Notice how you don't need to look at the package source code to understand what the packages do. The package has been written, and you don't care about how they work, but you use them so you only need to know how to use them. This is exactly what OOP is about.

I don't care how the shopping cart class adds an item to the cart, I just want to create a new cart and add something to it.

What you are doing however is diving into the source code as a tool to understand how OOP works.

Further to this, and probably more importantly, for web application development, I would research the MVC design pattern.

The MVC design Pattern stands for Model, View, Controller. Where in the case of a web application, The Model is responsible for modelling the database, the view is responsible for displaying content to the user. The controller is responsible for interacting with the model and handling user input.

I then think you should try to install the Laravel Framework or other "decent modern framework" on your local machine. Why do I say modern, because modern frameworks require a minumum PHP version of 5.3+ which mean that the PHP on your machine would support real OOP similar to that which you would get from the likes of Java.

There are many tutorials which will show you how to build web applications in laravel. Immediately, you will see that when you create a controller, you extend a BaseController. When you create a Model, you extend Eloquent. This means that you will already be using Polymorphism in your code. You will see that by using classes, they are being encapsulated, and you will see that each class has a specific role.

When you would like to interact with the database, you will initially create a new Model object within the controller methods. As you start to learn more, you will start learning how to inject dependencies into the controller, then learning how to dump your models and create repositories and program to interfaces instead.

A decent book on learning Laravel for beginners would be https://leanpub.com/codebright by Dale Rees. I met Dale at a Laravel meetup about 2 weeks ago.

Further to this, as you become more proficient building web applications, you will start to learn how to apply the following principles to your programming:

Single Responsibility Principle
Open Closed Principle
Liskov Substitution Principle
Interface Segragation Principle
Dependency Inversion Principle

As astropanic said, you could take a look at the source code of a good PHP framework or library. I recommend Zend Framework, it's very modular and has a great, professional design. I would say it is a very good piece of object-oriented PHP code.

Still, I think it's not that easy to learn from a huge piece of production code, since it wasn't really made to teach you anything. But if you want real-world object-oriented PHP code, the Zend Framework (or Symfony, or maybe CakePHP) is probably the way to go.

Purchase a book like "PHP and Mysql everyday apps for Dummies".

Its old I know [2005] but it shows concepts of User Logins, Forum, Shopping Carts, etc in both Procedural and Object Oriented with Mysqli.

It helped me learn Object Oriented PHP, I studied it a lot. Well worth the money.

OOP is much like grouping bits of your program into reuseable pieces. Its not that hard to be honest with you its just the idea of packing your functions into classes.

Real world mini example of OOP stuff below:

CLASS DATABASE
CLASS SESSIONS
CLASS WEBFORMS
CLASS EMAIL

CLASS ACCOUNTS (Example Functions below)
FUNCTION SELECTACCOUNT
FUNCTION CHECKPASSWORD
FUNCTION CHECKUSERNAME
FUNCTION CREATEACCOUNT

I hope you keep at it, PHP 6 will be re-engineered to support OOP more than ever.

Good Luck!

PHP: Real world OOP example

Tags:

Php

Oop

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