Readonly properties in PHP - Mini Project: Build & Apply

Readonly properties

📖 Scenario: Imagine you are creating 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 title and author should not change.

🎯 Goal: You will create a PHP class with readonly properties for title and author. Then, you will create an object of this class and display its properties.

📋 What You'll Learn

Create a class called Book with readonly properties title and author

Add a constructor to set the values of title and author

Create an object of the Book class with title '1984' and author 'George Orwell'

Print the title and author of the book

💡 Why This Matters

🌍 Real World

Readonly properties are useful when you want to store data that should not change after creation, like IDs, names, or fixed settings.

💼 Career

Understanding readonly properties helps you write safer and more reliable code, which is important in professional PHP development.

1

Create the Book class with readonly properties

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

PHP

<?php

// Create class Book with readonly properties title and author

Hint

Use the readonly keyword before the property type and name.

2

Add a constructor to set the readonly properties

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

PHP

<?php

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

    // Add constructor here

}

Hint

The constructor sets the readonly properties only once when the object is created.

3

Create a Book object with title and author

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

PHP

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

// Create a Book object with title '1984' and author 'George Orwell'

Hint

Use the new keyword to create the object and pass the exact strings.

4

Print the book's title and author

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

PHP

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

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

// Print the book's title and author

Hint

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

Practice

1. What is the main purpose of readonly properties in PHP?

easy

A. To store values that cannot be changed after being set

B. To allow properties to be changed anytime

C. To make properties private

D. To automatically generate getter methods

Solution

1. Step 1: Understand readonly property behavior

   Readonly properties in PHP are designed to hold values that can only be assigned once and cannot be modified later.

2. Step 2: Compare options with this behavior

   To store values that cannot be changed after being set correctly describes this behavior, while others describe unrelated features.

3. Final Answer:

   To store values that cannot be changed after being set -> Option A

4. Quick Check:

   Readonly means unchangeable after set = A [OK]

Hint: Readonly means value set once and then fixed [OK]

Common Mistakes:

• Thinking readonly allows changes anytime
• Confusing readonly with private visibility
• Assuming readonly auto-generates methods

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

easy

A. readonly public int $id;

B. public readonly int $id;

C. public int readonly $id;

D. int public readonly $id;

Solution

1. Step 1: Recall PHP property declaration order

   In PHP, visibility (public/private/protected) comes first, then readonly keyword, then type and variable name.

2. Step 2: Match syntax with options

   public readonly int $id; matches the correct order: public readonly int $id;. Others have incorrect order.

3. Final Answer:

   public readonly int $id; -> Option B

4. Quick Check:

   Visibility then readonly then type = B [OK]

Hint: Visibility keyword comes before readonly in declaration [OK]

Common Mistakes:

• Placing readonly before visibility
• Putting readonly after type
• Mixing order of keywords

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

class User {
    public readonly string $name;
    public function __construct(string $name) {
        $this->name = $name;
    }
}
$user = new User('Alice');
echo $user->name;
$user->name = 'Bob';
echo $user->name;

medium

A. Alice followed by a fatal error

B. BobBob

C. AliceBob

D. Syntax error

Solution

1. Step 1: Understand readonly property assignment

   The readonly property $name is assigned once in the constructor to 'Alice'. This is allowed.

2. Step 2: Analyze second assignment

   Trying to assign $user->name = 'Bob'; after construction causes a fatal error because readonly properties cannot be changed after initial assignment.

3. Final Answer:

   Alice followed by a fatal error -> Option A

4. Quick Check:

   Readonly blocks changes after set = C [OK]

Hint: Readonly allows one assignment only, later changes cause error [OK]

Common Mistakes:

• Assuming readonly properties can be changed anytime
• Expecting no error on second assignment
• Confusing syntax error with runtime error

4. Identify the error in the following PHP code using readonly properties:

class Config {
    public readonly string $version;
    public function __construct() {
        $this->version = '1.0';
        $this->version = '2.0';
    }
}

medium

A. No error, code is correct

B. Missing visibility keyword for property

C. Readonly keyword used incorrectly with string type

D. Assigning readonly property twice in constructor

Solution

1. Step 1: Check property assignments in constructor

   The readonly property $version is assigned twice: first '1.0', then '2.0'.

2. Step 2: Understand readonly assignment rules

   Readonly properties can only be assigned once, even inside the constructor. Assigning twice causes an error.

3. Final Answer:

   Assigning readonly property twice in constructor -> Option D

4. Quick Check:

   Readonly assigned once only = A [OK]

Hint: Assign readonly property only once, even in constructor [OK]

Common Mistakes:

• Assigning readonly property multiple times
• Ignoring visibility keyword importance
• Thinking readonly disallows string type

5. Given this PHP class with readonly properties, which code correctly creates an immutable user with an ID and name?

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

Choose the correct usage:

hard

A. $user = new User(1, 'John'); $user->name = 'Jane';

B. $user = new User(1); $user->name = 'John';

C. $user = new User(1, 'John'); // no changes after construction

D. $user = new User(); $user->id = 1; $user->name = 'John';

Solution

1. Step 1: Check constructor usage

   The constructor requires two parameters: int $id and string $name. $user = new User(1, 'John'); // no changes after construction correctly calls the constructor with both.

2. Step 2: Verify immutability

   Readonly properties cannot be changed after construction. $user = new User(1, 'John'); // no changes after construction respects this by not modifying properties after creation.

3. Final Answer:

   $user = new User(1, 'John'); // no changes after construction -> Option C

4. Quick Check:

   Construct with all params, no changes after = D [OK]

Hint: Call constructor fully, then do not modify readonly properties [OK]

Common Mistakes:

• Trying to assign readonly properties after construction
• Calling constructor with missing arguments
• Assuming properties can be set without constructor