Constructor promotion in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class using constructor promotion

What is the output of this PHP code using constructor promotion?

PHP

<?php
class User {
    public function __construct(
        public string $name,
        public int $age
    ) {}
}
$user = new User("Alice", 30);
echo $user->name . ' is ' . $user->age . ' years old.';

A Alice is years old.

B Alice is 30 years old.

C Error: Property age is not defined

D 30 is Alice years old.

💡 Hint

Constructor promotion automatically creates and assigns properties.

The constructor promotion syntax creates public properties $name and $age and assigns the passed values. So accessing $user->name and $user->age works as expected.

❓ Predict Output

intermediate

Output when mixing promoted and non-promoted properties

What will this PHP code output?

PHP

<?php
class Product {
    public string $category;
    public function __construct(
        public string $name,
        public float $price
    ) {
        $this->category = "General";
    }
}
$p = new Product("Book", 12.5);
echo $p->name . ' costs $' . $p->price . ' in category ' . $p->category;

A Book costs $12.5 in category General

B Book costs $ in category General

C Error: Property category is not initialized

D 12.5 costs $Book in category General

💡 Hint

Non-promoted properties must be initialized manually.

The property $category is declared but not promoted, so it is initialized inside the constructor. The promoted properties $name and $price are automatically assigned.

🔧 Debug

advanced

Identify the error in constructor promotion usage

What error does this PHP code produce?

PHP

<?php
class Car {
    public function __construct(
        private string $make,
        protected int $year = 2020,
        public $model
    ) {}
}
$car = new Car("Toyota", 2018, "Corolla");
echo $car->make;

A Notice: Undefined property: Car::$make

B Parse error: Default value for parameter must be constant

C Fatal error: Cannot access private property Car::$make

D No error, outputs: Toyota

💡 Hint

Private properties cannot be accessed directly outside the class.

The property $make is private, so trying to echo $car->make outside the class causes a fatal error due to visibility.

📝 Syntax

advanced

Which option causes a syntax error in constructor promotion?

Which of these constructor declarations will cause a syntax error in PHP?

A public function __construct(public int $x = 5, private string $y) {}

B public function __construct(public int $x, private string $y = "default") {}

C public function __construct(public int $x, private string $y) {}

D public function __construct(public int $x = 5, private string $y = "default") {}

💡 Hint

In PHP, parameters with default values must come after parameters without defaults.

Option A has a parameter with a default value ($x) before a parameter without a default ($y), which is invalid syntax.

🚀 Application

expert

Count properties created by constructor promotion

How many properties does this PHP class have after construction?

PHP

<?php
class Employee {
    public string $department = "HR";
    public function __construct(
        public string $name,
        private int $id,
        protected float $salary = 50000.0
    ) {}
}
$emp = new Employee("John", 1234);

A 3

B 5

C 2

D 4

💡 Hint

Count all declared properties including promoted and non-promoted.

The class has one non-promoted property $department and three promoted properties: $name, $id, $salary. Total is 4 properties.