Readonly classes in PHP - Mini Project: Build & Apply

Readonly Classes in PHP

📖 Scenario: You are building a simple system to store information about books in a library. Each book has a title and an author. Once a book is created, its details should not change.

🎯 Goal: Create a readonly class called Book with two properties: title and author. Initialize these properties in the constructor and then display the book details.

📋 What You'll Learn

Create a readonly class named Book

Add two public properties: title and author

Initialize these properties using a constructor

Create an instance of Book with specific values

Print the book's title and author

💡 Why This Matters

🌍 Real World

Readonly classes are useful when you want to create objects that should not change after creation, such as configuration settings, fixed data records, or value objects.

💼 Career

Understanding readonly classes helps you write safer and more predictable code, which is important in professional PHP development to avoid bugs and maintain data integrity.

1

Create the readonly class with properties

Create a readonly class called Book with two public properties: title and author.

PHP

<?php

// Define the readonly class Book with public properties title and author
// Your code here

Hint

Use the readonly keyword before class. The properties are implicitly readonly.

2

Add the constructor to initialize properties

Add a constructor method __construct to the Book class that takes two parameters: string $title and string $author. Inside the constructor, assign these parameters to the readonly properties $title and $author.

PHP

<?php

readonly class Book {
    public string $title;
    public string $author;

    // Add the constructor here
    // Your code here
}

Hint

Use public function __construct(string $title, string $author) and assign the parameters to the properties.

3

Create an instance of the readonly class

Create a variable called $book and assign it a new instance of the Book class with the title '1984' and author 'George Orwell'.

PHP

title = $title;
        $this->author = $author;
    }
}

// Create a new Book instance with title '1984' and author 'George Orwell'
// Your code here

Hint

Use new Book('1984', 'George Orwell') and assign it to $book.

4

Print the book details

Print the book's title and author using echo in the format: Title: 1984, Author: George Orwell.

PHP

title = $title;
        $this->author = $author;
    }
}

$book = new Book('1984', 'George Orwell');

// Print the book details below
// Your code here

Hint

Use echo and access the properties with $book->title and $book->author.

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