Constructor promotion in PHP - Step-by-Step Execution

Concept Flow - Constructor promotion

Define class with promoted properties

↓

Create object with constructor args

↓

Properties auto-assigned

↓

Use object properties directly

Constructor promotion lets you declare and assign class properties directly in the constructor parameters, simplifying code.

Execution Sample

PHP

<?php
class User {
  public function __construct(public string $name, public int $age) {}
}
$user = new User("Anna", 30);
echo $user->name . ', ' . $user->age;
?>

This code creates a User object with name and age using constructor promotion, then prints the values.

Execution Table

Step	Action	Evaluation	Result
1	Define class User with promoted properties	public string $name, public int $age	Properties declared and constructor created
2	Create new User object with args ('Anna', 30)	$name = 'Anna', $age = 30	Object $user created
3	Assign constructor args to properties	$this->name = 'Anna', $this->age = 30	Properties set automatically
4	Access $user->name	Returns 'Anna'	Output: Anna
5	Access $user->age	Returns 30	Output: 30
6	Concatenate and echo	'Anna, 30'	Printed: Anna, 30

💡 End of script after printing user name and age

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
$name	undefined	'Anna'	'Anna'	undefined
$age	undefined	30	30	undefined
$user->name	undefined	undefined	'Anna'	'Anna'
$user->age	undefined	undefined	30	30

Key Moments - 3 Insights

Why don't we see property declarations inside the class body?

Because constructor promotion declares and assigns properties directly in the constructor parameters, so separate declarations are not needed (see Step 1 and Step 3).

How are the constructor arguments assigned to properties automatically?

The 'public' keyword before parameters tells PHP to create properties and assign values automatically during object creation (Step 3 shows this).

Can we access properties like $user->name right after creating the object?

Yes, because properties are set during construction, so accessing them after Step 3 returns the assigned values.

Visual Quiz - 3 Questions

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

A 30

B undefined

C 0

D null

💡 Hint

Check the 'Result' column in Step 3 where properties are assigned.

At which step are the constructor arguments assigned to the object properties?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look for the step describing property assignment in the execution table.

If we remove 'public' from the constructor parameters, what changes in the execution?

A Object creation will fail

B Properties won't be created or assigned automatically

C Properties will still be created automatically

D Constructor will not run

💡 Hint

Recall the role of 'public' in constructor promotion from the key moments.

Concept Snapshot

Constructor promotion in PHP lets you declare and assign class properties directly in the constructor parameters.
Use visibility keywords (public/private/protected) before parameters.
This removes the need for separate property declarations and assignments.
Example: __construct(public string $name, public int $age) {}
Properties are set automatically when creating an object.

Full Transcript

Constructor promotion is a PHP feature that simplifies class property declaration and assignment. Instead of declaring properties separately and assigning them inside the constructor, you write the properties directly in the constructor parameters with visibility keywords like public. When you create an object, PHP automatically creates those properties and assigns the passed values. For example, class User with __construct(public string $name, public int $age) {} creates properties name and age and sets them when you do new User('Anna', 30). You can then access $user->name and $user->age directly. This reduces boilerplate code and makes classes cleaner.