Classes and Objects

A cookie cutter isn't a cookie. It's the shape that every cookie is made from. Each cookie you cut is its own cookie: you can frost one pink and one blue.

In Associative Arrays, you stored a student as ["name" => "Maria", "grade" => 88], and wrote separate functions to work with records like that. That works, but nothing connects the data to the functions, and nothing stops a typo like $student["grdae"].

Object-oriented programming (OOP) bundles data and the functions that use it into one thing: an object. It's how most modern PHP is written, including frameworks like Laravel.

Cookie cutter and cookies

• A class is the cookie cutter: a blueprint that describes what something has and what it can do.
• An object is a cookie: one actual thing made from the class. You can make as many as you like, and each one is separate.

<?php
declare(strict_types=1);

class Pet
{
    public string $name = "";
    public string $sound = "";

    public function speak(): string
    {
        return "$this->name says $this->sound!";
    }
}

$dog = new Pet();
$dog->name = "Bantay";
$dog->sound = "Woof";

$cat = new Pet();
$cat->name = "Mingming";
$cat->sound = "Meow";

echo $dog->speak(), "\n";
echo $cat->speak(), "\n";

Bantay says Woof!
Mingming says Meow!

Let's take it apart.

Defining a class

• class Pet starts the blueprint. Class names use PascalCase: each word starts with a capital, like Pet, ShoppingCart, BankAccount.
• By convention, the opening { of a class and of its methods goes on its own line. (That's the common PHP style, called PSR-12.)
• public string $name = ""; declares a property: a variable that every object of this class has. It has a type, like a function parameter, and a starting value.
• public function speak() declares a method: a function that belongs to the class.
• public means the property or method can be used from outside the class. The next lesson shows why you'd sometimes choose private instead.

Making objects: new

new Pet() makes a new object from the class, and gives it back. $dog and $cat are two separate objects, each with its own name and sound.

The arrow: ->

The arrow -> reaches inside an object:

• $dog->name is the name property of $dog. Notice there's no $ before name.
• $dog->speak() calls the speak method on $dog.

$this: "the object I belong to"

Inside a method, $this means "the object this method was called on." When you call $dog->speak(), $this is $dog, so $this->name is "Bantay". Call $cat->speak(), and the very same code reads "Mingming".

That's the key idea: one method, written once in the class, works with whichever object it's called on.

Methods that change the object

Methods can change their object's properties, too:

<?php
declare(strict_types=1);

class Counter
{
    public int $count = 0;

    public function increment(): void
    {
        $this->count++;
    }
}

$clicks = new Counter();
$likes = new Counter();

$clicks->increment();
$clicks->increment();
$likes->increment();

echo "Clicks: $clicks->count, likes: $likes->count\n";

Clicks: 2, likes: 1

Each object keeps its own count, just like each cookie keeps its own frosting.

Typed properties catch mistakes

Because the properties have types, PHP catches wrong values:

<?php
declare(strict_types=1);

class Counter
{
    public int $count = 0;
}

$c = new Counter();
$c->count = "many";
// error: Cannot assign string to property Counter::$count of type int

Counter::$count is how PHP names "the $count property of the Counter class."

And since the class lists its properties, your code editor can suggest them as you type, and warn you about a typo like $c->cuont. That's a big step up from array keys, which could be anything.

Looking inside an object

var_dump and print_r work on objects, too:

<?php
class Book
{
    public string $title = "Noli Me Tangere";
    public int $pages = 438;
}

print_r(new Book());

Book Object
(
    [title] => Noli Me Tangere
    [pages] => 438
)

Try it

A school library tracks its books as objects. Predict each line.

Running PHP here

Running PHP right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have PHP installed on your computer (see Setting Up), save it as index.php and run it with php index.php.

Check your prediction

Borrowed Noli Me Tangere. 1 left.
Borrowed El Filibusterismo. 0 left.
Borrowed Noli Me Tangere. 0 left.
No copies of Noli Me Tangere left.
El Fili available: 0

Each book tracks its own borrowed count. The Noli has 2 copies, so the third attempt finds none left. borrow calls available on the same object, using $this.

Try it yourself

1. Add a method giveBack(): void that lowers borrowed by 1, and use it on the Noli before the last borrow.
2. Add an author property, and change borrow's message to include it.
3. Try $noli->copies = "two";. What does PHP say, and why?

Check your understanding

What's the difference between a class and an object?

They are the same thing

An object is the blueprint; a class is made from it

A class can only have one object

A class is the blueprint; an object is one thing made from it

Inside a method, what does $this refer to?

The object the method was called on

The class itself

The first object ever created

The current file

How do you read the title property of the object in $book?

$book["title"]

$book.title

$book->title

$book->$title

Up next

Setting every property by hand after new is tedious, and nothing stops someone setting borrowed to -50. Next, you'll give your classes a proper setup step and protect their data, in Constructors and Visibility.