Readonly properties in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of readonly property assignment

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

PHP

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

A Alice

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

C null

D Parse error: syntax error, unexpected 'readonly'

💡 Hint

Readonly properties can be assigned once, usually in the constructor.

The readonly property $name is assigned in the constructor. Accessing it later prints the assigned value.

❓ Predict Output

intermediate

Error on modifying readonly property after construction

What happens when you try to modify a readonly property after the object is created?

PHP

<?php
class Point {
    public readonly int $x;
    public function __construct(int $x) {
        $this->x = $x;
    }
}
$p = new Point(5);
$p->x = 10;
echo $p->x;
?>

A 10

B Fatal error: Cannot modify readonly property Point::$x

C 5

D Parse error: syntax error

💡 Hint

Readonly properties cannot be changed after initial assignment.

Trying to assign a new value to a readonly property after construction causes a fatal error.

🧠 Conceptual

advanced

Readonly property initialization timing

When can a readonly property be assigned a value in PHP?

A Only once, but anywhere in the class methods

B Multiple times anywhere in the class methods

C Only once, and only inside the constructor or at property declaration

D Only at the moment of object destruction

💡 Hint

Think about when readonly properties get their value safely.

Readonly properties must be assigned once, either directly when declared or inside the constructor. After that, they cannot be changed.

🔧 Debug

advanced

Identify the error with readonly property usage

What error will this code produce?

PHP

<?php
class Config {
    public readonly array $settings = [];
    public function addSetting(string $key, string $value): void {
        $this->settings[$key] = $value;
    }
}
$config = new Config();
$config->addSetting('theme', 'dark');
print_r($config->settings);
?>

A Empty array output

B Array ( [theme] => dark )

C Parse error: syntax error, unexpected '['

D Fatal error: Cannot modify readonly property Config::$settings

💡 Hint

Readonly means the property cannot be changed after initialization, including its contents.

Even though the array property is assigned an empty array at declaration, modifying its contents later is not allowed because the property is readonly.

🚀 Application

expert

Using readonly properties with nested objects

Consider this code. What will be the output?

PHP

<?php
class Address {
    public readonly string $city;
    public function __construct(string $city) {
        $this->city = $city;
    }
}
class Person {
    public readonly Address $address;
    public function __construct(Address $address) {
        $this->address = $address;
    }
}
$addr = new Address('Paris');
$person = new Person($addr);
$person->address->city = 'London';
echo $person->address->city;
?>

A Fatal error: Cannot modify readonly property Address::$city

B Paris

C London

D Fatal error: Cannot modify readonly property Person::$address

💡 Hint

Readonly applies to the property itself, but what about the object it points to?

The property $address in Person is readonly, so it cannot be reassigned. But the Address object itself is mutable unless its own properties are readonly. Here, city is readonly, so trying to change it causes a fatal error.