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.

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