Readonly classes in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a readonly class in PHP.

PHP

<?php
[1] class User {
    public string $name;
}

Drag options to blanks, or click blank then click option'

Areadonly

Bfinal

Cabstract

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' instead of 'readonly' which prevents inheritance but not property changes.

Using 'static' which is unrelated to readonly properties.

In PHP 8.2+, the readonly keyword declares a class whose properties cannot be changed after initialization.

2fill in blank

medium

Complete the code to declare a readonly property in a normal class.

PHP

<?php
class Product {
    public [1] string $sku;
}

Drag options to blanks, or click blank then click option'

Areadonly

Bstatic

Cprivate

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' which means the property belongs to the class, not instance.

Using 'final' which is for methods or classes, not properties.

The readonly keyword before a property means it can only be assigned once, typically in the constructor.

3fill in blank

hard

Fix the error in the readonly class constructor to properly assign the readonly property.

PHP

<?php
readonly class Order {
    public function __construct([1] string $id) {
    }
}

Drag options to blanks, or click blank then click option'

Areadonly

Bprivate

Cpublic

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readonly' in constructor parameters which is invalid syntax.

Omitting visibility keyword causing property not to be assigned.

The constructor parameter should be declared with visibility like public to allow property promotion and assignment.

4fill in blank

hard

Fill the blank to create a readonly class with a readonly property using constructor property promotion.

PHP

<?php
[1] class Customer {
    public function __construct(public string $email) {}
}

Drag options to blanks, or click blank then click option'

Afinal

Breadonly

Cstatic

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' or 'static' instead of 'readonly' for the class.

Thinking you need readonly before the property type (it's invalid in readonly classes).

Declare the class readonly. Promoted properties are implicitly readonly in readonly classes.

5fill in blank

hard

Fill the blank to create a readonly class with two readonly properties and a method returning a property.

PHP

<?php
[1] class Invoice {
    public function __construct(public int $number, public float $amount) {}

    public function getAmount(): float {
        return $this->amount;
    }
}

Drag options to blanks, or click blank then click option'

Astatic

Bfinal

Cabstract

Dreadonly

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' instead of 'readonly' for the class.

Believing you need readonly keywords before each property type in the constructor.

Declaring the class readonly ensures all properties, including the two promoted ones, are readonly and immutable after creation.

Practice

1. What does a readonly class in PHP ensure about its properties?

easy

A. Properties are static and shared between all instances.

B. Properties can be changed anytime after creation.

C. All properties cannot be changed after the object is created.

D. Properties are private and cannot be accessed outside the class.

Solution

1. Step 1: Understand the meaning of readonly class

   A readonly class means all its properties are readonly by default, so they cannot be changed after the object is created.

2. Step 2: Compare options with this definition

   Only the option stating "All properties cannot be changed after the object is created." correctly states that properties cannot be changed after creation.

3. Final Answer:

   All properties cannot be changed after the object is created. -> Option C

4. Quick Check:

   Readonly class = immutable properties [OK]

Hint: Readonly means no property changes after creation [OK]

Common Mistakes:

• Thinking readonly means private or static
• Assuming properties can be changed anytime
• Confusing readonly with access modifiers

2. Which of the following is the correct syntax to declare a readonly class in PHP?

easy

A. readonly class User { public readonly string $name; }

B. class readonly User {}

C. class User readonly {}

D. readonly class User {}

Solution

1. Step 1: Recall PHP readonly class syntax

   In PHP, the keyword readonly comes before class to declare a readonly class.

2. Step 2: Check each option

   readonly class User {} correctly uses readonly class User {}. class readonly User {} reverses the order, class User readonly {} places readonly incorrectly, and readonly class User { public readonly string $name; } is invalid syntax.

3. Final Answer:

   readonly class User {} -> Option D

4. Quick Check:

   Keyword readonly before class [OK]

Hint: Put readonly before class keyword [OK]

Common Mistakes:

• Placing readonly after class name
• Using readonly as a property only
• Mixing readonly with access modifiers

3. What will be the output of this PHP code?

readonly class Point {
    public int $x;
    public int $y;
    public function __construct(int $x, int $y) {
        $this->x = $x;
        $this->y = $y;
    }
}

$p = new Point(5, 10);
echo $p->x . ', ' . $p->y;

medium

A. 5, 10

B. Error: Cannot modify readonly property

C. 0, 0

D. 5 10

Solution

1. Step 1: Understand readonly class property initialization

   Readonly properties can be set inside the constructor. Here, $x and $y are set to 5 and 10 respectively.

2. Step 2: Check output of echo statement

   The echo prints 5, 10 as expected because the properties were initialized correctly and not modified later.

3. Final Answer:

   5, 10 -> Option A

4. Quick Check:

   Readonly properties set in constructor output values [OK]

Hint: Readonly properties can be set in constructor only [OK]

Common Mistakes:

• Thinking readonly properties cannot be set at all
• Expecting error on property access
• Confusing output format with space instead of comma

4. Identify the error in this PHP code using a readonly class:

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

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

medium

A. Cannot assign to readonly property after construction

B. Missing readonly keyword before property declaration

C. Constructor must be static in readonly class

D. No error, code runs fine

Solution

1. Step 1: Understand readonly property behavior

   In a readonly class, properties can only be assigned once during construction. Changing them later causes an error.

2. Step 2: Analyze the code

   The code tries to change $user->name after construction, which is not allowed and causes an error.

3. Final Answer:

   Cannot assign to readonly property after construction -> Option A

4. Quick Check:

   Readonly properties are immutable after construction [OK]

Hint: Readonly properties cannot be changed after constructor [OK]

Common Mistakes:

• Thinking readonly properties can be changed anytime
• Confusing readonly class with readonly property keyword
• Ignoring error on property reassignment

5. Given this readonly class, which code snippet correctly creates an immutable user and tries to update the name without error?

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

Choose the correct approach:

hard

A. $user = new User('Alice');
$user->name = 'Bob';

B. $user = new User('Alice');
$newUser = new User('Bob');

C. $user = new User('Alice');
$user->name = strtoupper($user->name);

D. $user = new User('Alice');
unset($user->name);

Solution

1. Step 1: Understand immutability of readonly classes

   Readonly class properties cannot be changed after construction, so to 'change' data, create a new object with new values.

2. Step 2: Evaluate each option

   $user = new User('Alice'); $user->name = 'Bob'; tries to change property directly - error. $user = new User('Alice'); $user->name = strtoupper($user->name); tries to modify property - error. $user = new User('Alice'); $newUser = new User('Bob'); creates a new User object with new name - correct. $user = new User('Alice'); unset($user->name); tries to unset property - invalid.

3. Final Answer:

   Create a new User object with updated name -> Option B

4. Quick Check:

   Immutable means create new object for changes [OK]

Hint: Change readonly data by creating new object [OK]

Common Mistakes:

• Trying to modify readonly property directly
• Using unset on readonly properties
• Assuming readonly means properties can be changed