Why Readonly classes in PHP? - Purpose & Use Cases

The Big Idea

What if your data could protect itself from accidental changes without extra code?

The Scenario

Imagine you have a class representing a user profile, and you want to make sure once the profile is created, its data never changes accidentally throughout your program.

The Problem

Manually checking and preventing changes to each property everywhere in your code is slow and error-prone. You might forget to add checks, leading to bugs that are hard to find.

The Solution

Readonly classes let you declare that all properties cannot be changed after creation, so PHP enforces this automatically, saving you from writing extra code and preventing accidental changes.

Before vs After

✗ Before

$user->name = 'Alice';
// Later in code
$user->name = 'Bob'; // Oops, changed by mistake!

✓ After

readonly class User {
  public string $name;
  public function __construct(string $name) {
    $this->name = $name;
  }
}
// Now, trying to change $user->name will cause an error

What It Enables

You can trust your objects to stay consistent and unchanged, making your code safer and easier to understand.

Real Life Example

In a banking app, account details like account number or creation date should never change after creation. Readonly classes ensure this data stays fixed.

Key Takeaways

Readonly classes prevent accidental changes to object properties.

They reduce bugs by enforcing immutability automatically.

They make your code safer and easier to maintain.

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