PHP Classes/Objects

PHP Classes and Objects: Defining Blueprints and Instances

A PHP class is a blueprint defining properties (data) and methods (functions), while an object is a concrete instance created from that class using the new keyword. Inside a class method, the $this pseudo-variable refers to the current object instance.

Defining a Class and Creating Objects

Modern PHP supports typed properties, strict method signatures, and readonly properties (PHP 8.1+) that cannot be overwritten after initialization.

<?php
declare(strict_types=1);

class Product
{
    public string $name;
    public float $price;
    private int $stock = 0;

    public function setStock(int $quantity): void
    {
        if ($quantity < 0) {
            throw new InvalidArgumentException('Stock cannot be negative.');
        }
        $this->stock = $quantity;
    }

    public function isAvailable(): bool
    {
        return $this->stock > 0;
    }
}

$keyboard = new Product();
$keyboard->name = 'Compact Mechanical Keyboard';
$keyboard->price = 79.00;
$keyboard->setStock(12);

echo "{$keyboard->name} available: " . ($keyboard->isAvailable() ? 'Yes' : 'No') . PHP_EOL;
var_dump($keyboard instanceof Product);

Reference Semantics and instanceof

Unlike PHP arrays and scalars—which copy on assignment—PHP variables hold object handles pointing to the same instance in memory. If you write $b = $a; and change $b->name, $a->name reflects that change too; use clone $a when you truly need an independent copy.

Use $item instanceof Product to verify an object belongs to a specific class or implements an interface before calling its methods.

Common Mistakes to Avoid

• Reading an uninitialized typed property: Accessing public string $name; before assigning a value throws an Error: Typed property must not be accessed before initialization. Initialize properties inline or in __construct().
• Using $this inside static methods: Static methods belong to the class, not an instance, so $this does not exist inside them.
• Cloning objects containing nested objects blindly: clone creates a shallow copy; implement __clone() if nested child objects also need duplicating.

Frequently Asked Questions

What does -> mean in PHP?

The object operator -> accesses a property or calls a method on an object instance, such as $user->getName().

Can I create multiple independent objects from one class?

Yes. Every new Product() call allocates a separate object instance with its own property values in memory.

What does readonly class do in PHP 8.2+?

Marking a class readonly automatically makes all its declared properties immutable once initialized in the constructor—great for value objects and DTOs.