Readonly classes in PHP - Step-by-Step Execution

Concept Flow - Readonly classes

Define readonly class

↓

Create instance with properties

↓

Try to modify property?

Yes→Error: Cannot modify readonly property

No↓

Use properties as read-only

↓

End

This flow shows how a readonly class is defined, instantiated, and how its properties cannot be changed after creation.

Execution Sample

PHP

<?php
readonly class User {
  public string $name;
  public function __construct(string $name) {
    $this->name = $name;
  }
}
$user = new User("Alice");
$user->name = "Bob"; // Error
?>

Defines a readonly class User with a name property, creates an instance, then tries to change the name property which causes an error.

Execution Table

Step	Action	Property 'name' Value	Result/Output
1	Define readonly class User	N/A	Class User is readonly
2	Create User instance with name='Alice'	name = 'Alice'	Instance created successfully
3	Attempt to assign name='Bob'	name = 'Alice'	Error: Cannot modify readonly property User::$name
4	End	name = 'Alice'	Execution stops due to error

💡 Execution stops at step 3 because readonly property cannot be modified after initialization

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
$user->name	undefined	Alice	Alice	Alice

Key Moments - 2 Insights

Why can't we change the property 'name' after creating the User object?

Because the class is declared as readonly, its properties can only be set once during construction. Step 3 in the execution_table shows the error when trying to modify 'name'.

Is it allowed to set the property 'name' inside the constructor?

Yes, setting properties inside the constructor is allowed and is how readonly properties get their initial value, as shown in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $user->name after step 3?

A "Alice"

B "Bob"

C undefined

D null

💡 Hint

Check the 'Property 'name' Value' column at step 3 in the execution_table.

At which step does the program stop due to an error?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Result/Output' column for the error message in the execution_table.

If the class was not declared readonly, what would happen at step 3?

A Error would still occur

B The program would crash

C The property 'name' would change to 'Bob'

D The property would become null

💡 Hint

Consider how normal class properties behave when assigned after construction.

Concept Snapshot

readonly class ClassName {
  public type $property;
  public function __construct(type $property) {
    $this->property = $property; // Allowed
  }
}
Properties cannot be changed after construction; attempts cause errors.

Full Transcript

This example shows how to define a readonly class in PHP. The class User has a readonly property 'name' set in the constructor. When we create a User object with name 'Alice', it works fine. But if we try to change 'name' later to 'Bob', PHP throws an error because readonly properties cannot be modified after initialization. The execution table traces these steps clearly, showing the error at step 3. The variable tracker confirms the property value stays 'Alice'. This helps beginners understand readonly classes by watching the code run step-by-step.