PHP OOP Basics Classes, Objects, Constructors: Tutorial, Examples, FAQs & Interview Tips

Object-Oriented Programming (OOP) organizes code into classes and objects. A class is a blueprint; an object is an instance of that class.

Classes, Objects, and Constructors

Use class to define a class, new to create an object, and __construct() as the constructor. Access members with -> and use $this to refer to the current object.

Class and Object

<?php
class Car {
    // Properties
    public string $make;
    public string $model;
    private int $speed = 0;

    // Constructor
    public function __construct(string $make, string $model) {
        $this->make  = $make;
        $this->model = $model;
    }

    // Methods
    public function accelerate(int $amount): void {
        $this->speed += $amount;
    }

    public function getSpeed(): int {
        return $this->speed;
    }

    // Destructor
    public function __destruct() {
        echo "{$this->make} {$this->model} destroyed\n";
    }

    // __toString magic method
    public function __toString(): string {
        return "{$this->make} {$this->model} @ {$this->speed} km/h";
    }
}

$car = new Car("Toyota", "Corolla");
$car->accelerate(60);
echo $car->getSpeed(); // 60
echo $car;             // Toyota Corolla @ 60 km/h
?>

Access Modifiers

public - accessible everywhere. protected - accessible within the class and subclasses. private - accessible only within the class.

Access Modifiers

<?php
class BankAccount {
    public string $owner;
    protected float $balance;
    private string $pin;

    public function __construct(string $owner, float $balance, string $pin) {
        $this->owner   = $owner;
        $this->balance = $balance;
        $this->pin     = $pin;
    }

    public function deposit(float $amount): void {
        $this->balance += $amount;
    }

    public function getBalance(): float {
        return $this->balance;
    }

    private function validatePin(string $pin): bool {
        return $this->pin === $pin;
    }

    public function withdraw(float $amount, string $pin): bool {
        if ($this->validatePin($pin) && $this->balance >= $amount) {
            $this->balance -= $amount;
            return true;
        }
        return false;
    }
}

$acc = new BankAccount("Alice", 1000.0, "1234");
$acc->deposit(500);
echo $acc->getBalance(); // 1500
echo $acc->withdraw(200, "1234") ? "Success" : "Failed"; // Success
?>

Static Properties and Methods

Static members belong to the class itself, not to any instance. Access them with :: (scope resolution operator).

Static Members

<?php
class Counter {
    private static int $count = 0;
    const VERSION = "1.0";

    public function __construct() {
        self::$count++;
    }

    public static function getCount(): int {
        return self::$count;
    }

    public static function reset(): void {
        self::$count = 0;
    }
}

$a = new Counter();
$b = new Counter();
$c = new Counter();

echo Counter::getCount();  // 3
echo Counter::VERSION;     // 1.0
Counter::reset();
echo Counter::getCount();  // 0
?>

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Key Takeaways

OOP in PHP uses classes (blueprints) and objects (instances). Define with class ClassName {}.
__construct() is the constructor - runs automatically when an object is created with new.
Access modifiers: public (anywhere), protected (class + subclasses), private (class only).
$this refers to the current object instance inside a class method.
Static properties/methods belong to the class, not instances - access with ClassName::method().
Magic methods: __toString(), __get(), __set(), __call(), __destruct().
PHP 8+ constructor promotion: public function __construct(public string $name, private int $age) {}

Common Mistakes to Avoid

WRONG class User { public $password; }

RIGHT class User { private string $password; }

Always make sensitive properties private. Expose them only through controlled getter/setter methods.

WRONG $obj->method = function(){};

RIGHT Define methods inside the class body

PHP does not support adding methods to objects at runtime like JavaScript. All methods must be defined in the class.

WRONG class God { /* everything */ }

RIGHT Split into User, Auth, Profile, etc.

The Single Responsibility Principle: each class should have one reason to change. Large "God classes" are hard to test and maintain.

Frequently Asked Questions

What is the difference between abstract class and interface in PHP?

Abstract class: can have implemented methods, properties, and constructor. A class can extend only one abstract class. Interface: only method signatures (no implementation). A class can implement multiple interfaces. Use interfaces for contracts, abstract classes for shared base behavior.

What are PHP traits?

Traits are reusable code blocks that can be included in multiple classes using use TraitName. They solve the problem of single inheritance by allowing code reuse across unrelated classes. Traits can have methods and properties.

What is late static binding in PHP?

Late static binding uses static:: instead of self::. self:: always refers to the class where the method is defined. static:: refers to the class that was called at runtime - useful in inheritance chains.

PHP OOP Basics Classes, Objects, Constructors: Tutorial, Examples, FAQs & Interview Tips

Classes, Objects, and Constructors

Access Modifiers

Static Properties and Methods

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