Readonly classes in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of readonly class property assignment

What will be the output of this PHP code using a readonly class?

PHP

<?php
readonly class User {
    public string $name;
    public function __construct(string $name) {
        $this->name = $name;
    }
}

$user = new User("Alice");
echo $user->name;
$user->name = "Bob";
echo $user->name;

A AliceBob

B Fatal error: Cannot declare readonly class User

C AliceAlice

D

Alice
Fatal error: Cannot modify readonly property User::$name

💡 Hint

Readonly classes allow properties to be assigned only once, typically in the constructor.

The readonly class User allows property $name to be set only once during construction. The first echo prints 'Alice'. The second assignment to $user->name causes a fatal error because the property is readonly.

🧠 Conceptual

intermediate

Behavior of readonly classes with mutable properties

Which statement about readonly classes in PHP is true?

A Readonly classes prevent reassignment of properties but do not prevent mutation of objects assigned to properties.

B Readonly classes only allow properties to be assigned in the constructor and nowhere else, including nested objects.

C Readonly classes allow modification of properties if they are objects, but not scalar values.

D Readonly classes prevent modification of all properties, including objects referenced by properties.

💡 Hint

Think about how readonly affects references to objects stored in properties.

Readonly classes prevent reassigning properties after initialization, but if a property holds an object, the object's internal state can still be changed because the reference itself is readonly, not the object.

🔧 Debug

advanced

Identify the error in readonly class property initialization

What error will this PHP code produce?

PHP

<?php
readonly class Product {
    public string $name = "Default";
    public function __construct() {
        $this->name = "Gadget";
    }
}

$product = new Product();
echo $product->name;

A Gadget

B Fatal error: Cannot initialize readonly property Product::$name twice

C Default

D Parse error: Unexpected token '='

💡 Hint

Readonly properties can only be assigned once, either at declaration or in constructor, not both.

Assigning a default value and then reassigning in the constructor causes a fatal error because the readonly property is initialized twice.

📝 Syntax

advanced

Valid syntax for declaring a readonly class

Which of the following PHP code snippets correctly declares a readonly class with a readonly property?

A

class Car readonly {
    public readonly string $model;
    public function __construct(string $model) {
        $this->model = $model;
    }
}

B

class readonly Car {
    public string $model readonly;
    public function __construct(string $model) {
        $this->model = $model;
    }
}

C

readonly class Car {
    public string $model;
    public function __construct(string $model) {
        $this->model = $model;
    }
}

D

readonly class Car {
    public readonly string $model;
    public function __construct(string $model) {
        $this->model = $model;
    }
}

💡 Hint

The readonly keyword must appear before class and property declarations correctly.

Option C correctly uses 'readonly class Car' where the property is implicitly readonly. Option C causes a compile-time error because properties cannot be declared explicitly readonly in readonly classes. Options A and B have invalid syntax.

🚀 Application

expert

Count properties in a readonly class after instantiation

Given this readonly class, how many properties does the object have after creation?

PHP

<?php
readonly class Book {
    public string $title;
    public string $author;
    public function __construct(string $title, string $author) {
        $this->title = $title;
        $this->author = $author;
    }
}

$book = new Book("1984", "Orwell");
$props = get_object_vars($book);
echo count($props);

A 2

B 1

C 0

D Fatal error: Cannot access properties of readonly class

💡 Hint

Readonly properties are still accessible and countable like normal properties.

Readonly properties are normal properties that cannot be reassigned after initialization. get_object_vars returns all accessible properties, so count is 2.