Readonly properties in PHP

Introduction

Readonly properties let you create values that can only be set once and never changed. This helps keep your data safe and predictable.

When you want to store a value that should not change after it is first set, like a user ID.

When you want to make sure some settings or configuration values stay the same throughout the program.

When you want to prevent accidental changes to important data in your objects.

When you want to clearly show which properties are meant to be fixed after creation.

Syntax

PHP

class ClassName {
    public readonly Type $propertyName;

    public function __construct(Type $value) {
        $this->propertyName = $value;
    }
}

The readonly keyword is placed before the property type and name.

You can only assign a value to a readonly property once, usually in the constructor.

Examples

This example shows a readonly property $id that can only be set when creating a User.

PHP

<?php
class User {
    public readonly int $id;

    public function __construct(int $id) {
        $this->id = $id;
    }
}

Readonly property $appName holds the application name and cannot be changed later.

PHP

<?php
class Config {
    public readonly string $appName;

    public function __construct(string $appName) {
        $this->appName = $appName;
    }
}

Sample Program

This program creates a Product with readonly properties name and price. It prints the product details. Trying to change price after creation will cause an error.

PHP

<?php
class Product {
    public readonly string $name;
    public readonly float $price;

    public function __construct(string $name, float $price) {
        $this->name = $name;
        $this->price = $price;
    }
}

$product = new Product("Book", 12.99);
echo "Product: {$product->name}\n";
echo 'Price: $' . $product->price . "\n";

// Trying to change readonly property will cause error
// $product->price = 15.99; // Uncommenting this line will cause an error

Important Notes

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

Trying to change a readonly property after it is set will cause a fatal error.

Readonly properties help make your code safer and easier to understand.

Summary

Readonly properties store values that cannot be changed after being set.

They are useful for fixed data like IDs or configuration.

Assign them once, usually in the constructor, and then they stay constant.

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