PHP Access Modifiers

PHP Access Modifiers: Controlling Visibility with public, protected, and private

PHP access modifiers—public, protected, and private—control which parts of your codebase can read or call a class's properties, methods, and constants. Encapsulating internal state behind clear visibility boundaries prevents outside code from breaking object invariants.

Comparing Visibility Levels in PHP

Choose the strictest visibility level that still lets collaborators use your class cleanly.

<?php
declare(strict_types=1);

class Order
{
    private array $items = [];
    protected float $taxRate = 0.08;

    public function addItem(string $name, float $price): void
    {
        if ($price < 0) {
            throw new InvalidArgumentException('Price cannot be negative.');
        }
        $this->items[] = ['name' => $name, 'price' => $price];
    }

    public function calculateTotal(): float
    {
        $subtotal = array_sum(array_column($this->items, 'price'));
        return round($subtotal * (1 + $this->taxRate), 2);
    }
}

$order = new Order();
$order->addItem('USB Hub', 30.00);
echo "Total with tax: $" . $order->calculateTotal() . PHP_EOL;

Because $items is private, external callers must go through addItem(), which guarantees negative prices can never sneak into the order total.

Combining Visibility with readonly

In PHP 8.1+, marking a property public readonly string $id lets outside code read $order->id directly without writing a boilerplate getId() getter, while still blocking external reassignments.

Common Mistakes to Avoid

• Leaving properties public out of habit: Mutable public properties let any caller bypass validation checks. Default to private or public readonly.
• Making helper methods public when only used internally: Internal formatting or parsing helpers belong in private methods so you can refactor them freely later without breaking callers.
• Forgetting visibility keywords on methods: Omitting public/private defaults a method to public, though PSR-12 coding standards require declaring visibility explicitly on every method and property.

Frequently Asked Questions

Can two instances of the same class access each other's private properties?

Yes. In PHP, visibility is enforced at the class level, not the object instance level—so $this->compare($otherOrder) can read $otherOrder->items.

When should I choose protected instead of private?

Use protected only when a property or hook method is intentionally designed to be overridden or read by subclasses. Otherwise, prefer private.

Can constants also have access modifiers in PHP?

Yes. Since PHP 7.1, class constants support public const, protected const, and private const.