PHP Constructor

PHP Constructor: Initializing Objects with __construct()

A PHP constructor is a special __construct() method that PHP calls automatically every time you instantiate an object with new. Constructors guarantee that every new object starts in a valid, initialized state with all required dependencies and values present.

Classic Constructor vs PHP 8 Property Promotion

Before PHP 8, developers had to declare every property three times: as a class property, a constructor parameter, and an assignment line. PHP 8 constructor property promotion combines all three into one clean parameter list by adding visibility (public, private, protected, or readonly) directly inside __construct().

<?php
declare(strict_types=1);

class UserAccount
{
    public function __construct(
        public readonly int $id,
        public string $email,
        private string $role = 'member'
    ) {
        if (!filter_var($this->email, FILTER_VALIDATE_EMAIL)) {
            throw new InvalidArgumentException('Invalid account email.');
        }
    }

    public function getRole(): string
    {
        return $this->role;
    }
}

$user = new UserAccount(id: 7, email: '[email protected]');
echo "User #{$user->id} ({$user->email}) - Role: {$user->getRole()}" . PHP_EOL;

Dependency Injection via Constructors

In clean Laravel and PHP services, constructors accept collaborators like repositories, mailers, or loggers. Constructor injection makes dependencies obvious and allows unit tests to pass fake or mocked collaborators effortlessly.

Common Mistakes to Avoid

• Performing heavy network or database calls inside __construct(): Keep constructors fast and deterministic; validate inputs and assign properties rather than firing HTTP requests on instantiation.
• Adding a return type on __construct(): Constructors never return values. Declaring : void on __construct() causes a fatal error in PHP.
• Forgetting parent::__construct() in subclasses: If a child class defines its own constructor, PHP does not call the parent constructor automatically unless you invoke parent::__construct(...).

Frequently Asked Questions

Can a PHP class have multiple constructors?

No. PHP supports only one __construct() method per class. To offer alternative creation paths, use named static factory methods like User::fromArray($row).

Can a constructor be private?

Yes. A private function __construct() prevents direct new ClassName() calls outside the class, commonly used with static factory methods or value objects.

Can I mix promoted and regular parameters in __construct()?

Yes. Any parameter without a visibility keyword (public, protected, private, or readonly) behaves as a normal constructor argument.