Readonly properties in PHP - Step-by-Step Execution

Concept Flow - Readonly properties

Create object

↓

Assign readonly property once

↓

Try to assign again?

Yes→Error: Cannot modify readonly property

No↓

Use property value

↓

End

Readonly properties in PHP allow setting a property only once during object creation or initialization. Any further changes cause an error.

Execution Sample

PHP

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

This code creates a User object with a readonly property 'name' set once after creation. Trying to change it later causes an error.

Execution Table

Step	Action	Property 'name' Value	Result
1	Create User object	Unset	Object created
2	Assign 'Alice' to readonly property 'name'	Alice	Assignment successful
3	Try to assign 'Bob' to readonly property 'name'	Alice	Error: Cannot modify readonly property
4	Use property 'name' value	Alice	Outputs 'Alice'

💡 Execution stops at step 3 due to error when trying to modify readonly property

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
$user->name	Unset	Unset	Alice	Alice (unchanged due to error)	Alice

Key Moments - 2 Insights

Why can't we change the value of a readonly property after it's set?

Readonly properties in PHP are designed to be assigned only once. As shown in execution_table step 3, trying to assign a new value causes an error, preventing modification.

When can we assign a value to a readonly property?

You can assign a value only once, typically during object creation or inside the constructor, as shown in step 2 where the property is set successfully.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $user->name after step 2?

A "Bob"

B "Alice"

C Unset

D Error

💡 Hint

Check the 'Property 'name' Value' column at step 2 in the execution_table.

At which step does the error occur when trying to modify the readonly property?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

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

If we remove the second assignment ($user->name = "Bob";), how does the execution table change?

A Step 3 is removed, no error occurs

B Step 2 causes an error

C Step 4 outputs 'Bob'

D Step 1 fails to create object

💡 Hint

Refer to execution_table rows and see what causes the error.

Concept Snapshot

Readonly properties in PHP:
- Declared with 'public readonly'
- Can be assigned only once (usually in constructor)
- Further assignments cause error
- Useful for immutable object data
- Helps prevent accidental changes

Full Transcript

This visual execution trace shows how readonly properties in PHP work. First, an object is created with a readonly property 'name'. The property is assigned once successfully after creation. When the code tries to assign a new value to the readonly property, PHP throws an error and stops execution. The variable tracker shows the property value remains unchanged after the error. Key moments clarify that readonly properties can only be set once and cannot be changed later. The quiz questions help reinforce understanding by asking about property values at different steps and the error occurrence.