PHP Tutorial · Object-Oriented PHP · Lesson 29 of 38

PHP Classes and Objects

Define classes with properties and methods, and create objects.

A class and its objects

A class defines properties (data) and methods (behaviour). new creates an object; -> accesses its members; $this refers to the current object:

Example 1Try it yourself

<?php
class Product
{
    public string $name;
    private float $price;

    public function setPrice(float $price): void
    {
        if ($price < 0) {
            throw new InvalidArgumentException("Price can't be negative");
        }
        $this->price = $price;
    }

    public function label(): string
    {
        return sprintf("%s — $%.2f", $this->name, $this->price);
    }
}

$mug = new Product();
$mug->name = "Mug";
$mug->setPrice(9.5);
echo $mug->label(), "\n";

Output

Mug — $9.50

Visibility

public — accessible everywhere.
protected — inside the class and its children.
private — inside the class only.

Keep properties private and expose behaviour through methods, so the object controls its own rules.

Readonly properties

A readonly property can be set once (in the constructor) and never changed — great for value objects:

Example 2Try it yourself

<?php
final class Money
{
    public function __construct(
        public readonly int $cents,
        public readonly string $currency = "USD",
    ) {}

    public function add(Money $other): Money
    {
        return new Money($this->cents + $other->cents, $this->currency);
    }
}

$total = (new Money(1999))->add(new Money(501));
echo $total->cents, " ", $total->currency, "\n";
try {
    $total->cents = 0;
} catch (Error $e) {
    echo $e->getMessage(), "\n";
}

Output

2500 USD
Cannot modify readonly property Money::$cents

Test yourself

Which operator accesses a property or method on an object?

.
->
::
=>

Show answer

Answer: ->