Constructor promotion in PHP

Introduction

Constructor promotion helps you write less code when creating classes. It lets you set up properties and their values quickly inside the constructor.

When you want to create a class with properties and set them right away.

When you want to avoid writing extra lines for declaring properties and assigning values.

When you want cleaner and shorter code for simple classes.

When you want to improve readability by combining property declaration and constructor parameters.

Syntax

PHP

class ClassName {
    public function __construct(private Type $property1, public Type $property2) {
        // no need to assign properties manually
    }
}

You declare properties directly in the constructor parameters using visibility keywords like public, private, or protected.

This feature was introduced in PHP 8.0.

Examples

This class has two properties, name and age, declared and assigned in the constructor.

PHP

<?php
class User {
    public function __construct(private string $name, public int $age) {}
}

Properties x and y have default values and are promoted in the constructor.

PHP

<?php
class Point {
    public function __construct(public float $x = 0.0, public float $y = 0.0) {}
}

Sample Program

This program creates a Product object using constructor promotion. It prints the product name and price.

PHP

<?php
class Product {
    public function __construct(public string $name, private float $price) {}

    public function getPrice(): float {
        return $this->price;
    }
}

$product = new Product("Book", 12.99);
echo "Product: " . $product->name . "\n";
echo "Price: $" . $product->getPrice() . "\n";

Important Notes

Constructor promotion only works with properties declared in the constructor parameters.

You can still add normal properties and methods outside the constructor.

Summary

Constructor promotion lets you declare and assign properties in one step inside the constructor.

It reduces code and improves readability for simple classes.

Use visibility keywords in constructor parameters to promote properties.

Practice

1. What is the main benefit of using constructor promotion in PHP classes?

easy

A. It automatically generates getter and setter methods for properties.

B. It allows declaring and assigning properties in one step inside the constructor.

C. It enforces strict typing on all class properties.

D. It makes the class immutable by default.

Solution

1. Step 1: Understand constructor promotion purpose

   Constructor promotion lets you declare and assign properties directly in the constructor parameters.

2. Step 2: Compare options

   Only It allows declaring and assigning properties in one step inside the constructor. correctly describes this benefit. Options B, C, and D describe unrelated features.

3. Final Answer:

   It allows declaring and assigning properties in one step inside the constructor. -> Option B

4. Quick Check:

   Constructor promotion = Declare and assign properties together [OK]

Hint: Constructor promotion combines property declaration and assignment [OK]

Common Mistakes:

• Thinking it auto-generates getters/setters
• Confusing with immutability
• Assuming it enforces typing

2. Which of the following is the correct syntax for constructor promotion in PHP?

easy

A. public function __construct(private string $name, int $age) {}

B. public function __construct(var $name, var $age) {}

C. public function __construct(string $name, int $age) { $this->name = $name; }

D. public function __construct(private string $name, private int $age) {}

Solution

1. Step 1: Identify visibility keywords in constructor parameters

   Constructor promotion requires visibility keywords like private, protected, or public before parameters.

2. Step 2: Check each option

   public function __construct(private string $name, private int $age) {} uses private for both parameters, which is correct. public function __construct(private string $name, int $age) {} misses visibility for $age. public function __construct(string $name, int $age) { $this->name = $name; } is normal assignment, not promotion. public function __construct(var $name, var $age) {} uses invalid 'var' keyword.

3. Final Answer:

   public function __construct(private string $name, private int $age) {} -> Option D

4. Quick Check:

   Visibility + parameter = constructor promotion [OK]

Hint: Use visibility keywords on constructor parameters for promotion [OK]

Common Mistakes:

• Omitting visibility keyword
• Using invalid keywords like var
• Confusing promotion with manual assignment

3. What will be the output of the following PHP code?

class User {
    public function __construct(private string $name, private int $age) {}
    public function getInfo() {
        return "$this->name is $this->age years old.";
    }
}
$user = new User('Alice', 30);
echo $user->getInfo();

medium

A. Alice is 30 years old.

B. Error: Cannot access private property

C. 30 is Alice years old.

D. Syntax error

Solution

1. Step 1: Understand property promotion and access

   The properties $name and $age are private but accessible inside class methods like getInfo().

2. Step 2: Analyze getInfo() output

   It returns a string combining $name and $age values. Since $user is created with 'Alice' and 30, output is "Alice is 30 years old."

3. Final Answer:

   Alice is 30 years old. -> Option A

4. Quick Check:

   Private properties accessible inside class = correct output [OK]

Hint: Private promoted properties accessible inside class methods [OK]

Common Mistakes:

• Assuming private properties are inaccessible inside class
• Mixing order of name and age in output
• Expecting syntax error

4. Identify the error in this PHP class using constructor promotion:

class Product {
    public function __construct(private $name, private int $price) {}
}

medium

A. Cannot use private keyword in constructor parameters.

B. Price parameter should be string, not int.

C. Missing type declaration for $name parameter.

D. Constructor promotion requires public visibility only.

Solution

1. Step 1: Check parameter declarations

   The $name parameter lacks a type declaration, which is required for constructor promotion to be clear and strict.

2. Step 2: Validate other options

   Private visibility is allowed, int type for price is valid, and promotion supports all visibility levels.

3. Final Answer:

   Missing type declaration for $name parameter. -> Option C

4. Quick Check:

   All promoted parameters need type declarations [OK]

Hint: All promoted parameters must have types declared [OK]

Common Mistakes:

• Omitting type for promoted parameters
• Thinking only public visibility allowed
• Confusing parameter types

5. You want to create a PHP class Book with promoted properties title (string), author (string), and pages (int). You also want to add a method summary() that returns a string like "Title by Author, Pages pages.". Which of the following is the correct complete class code?

hard

A. class Book {
public function __construct(public string $title, public string $author, public int $pages) {}
public function summary() {
return "$this->title by $this->author, $this->pages pages.";
}
}

B. class Book {
public string $title;
public string $author;
public int $pages;
public function __construct($title, $author, $pages) {
$this->title = $title;
$this->author = $author;
$this->pages = $pages;
}
public function summary() {
return "$title by $author, $pages pages.";
}
}

C. class Book {
public function __construct(string $title, string $author, int $pages) {}
public function summary() {
return "$this->title by $this->author, $this->pages pages.";
}
}

D. class Book {
public function __construct(private string $title, private string $author, private int $pages) {}
public function summary() {
return "$title by $author, $pages pages.";
}
}

Solution

1. Step 1: Check constructor promotion syntax

   class Book { public function __construct(public string $title, public string $author, public int $pages) {} public function summary() { return "$this->title by $this->author, $this->pages pages."; } } correctly uses public visibility with types in constructor parameters to promote properties.

2. Step 2: Verify summary() method access and string

   class Book { public function __construct(public string $title, public string $author, public int $pages) {} public function summary() { return "$this->title by $this->author, $this->pages pages."; } } uses $this->property to access promoted properties and returns the correct formatted string.

3. Step 3: Analyze other options

   class Book { public string $title; public string $author; public int $pages; public function __construct($title, $author, $pages) { $this->title = $title; $this->author = $author; $this->pages = $pages; } public function summary() { return "$title by $author, $pages pages."; } } uses manual assignment but summary() incorrectly uses variables without $this->. class Book { public function __construct(string $title, string $author, int $pages) {} public function summary() { return "$this->title by $this->author, $this->pages pages."; } } promotes parameters but does not assign them to properties. class Book { public function __construct(private string $title, private string $author, private int $pages) {} public function summary() { return "$title by $author, $pages pages."; } } uses private properties but summary() accesses variables without $this->, causing errors.

4. Final Answer:

   Option A code with public promoted properties and correct summary() -> Option A

5. Quick Check:

   Promotion + correct property access = class Book { public function __construct(public string $title, public string $author, public int $pages) {} public function summary() { return "$this->title by $this->author, $this->pages pages."; } } [OK]

Hint: Use visibility + $this->property in methods for promoted properties [OK]

Common Mistakes:

• Forgetting $this-> in methods
• Not assigning promoted properties properly
• Mixing private properties with wrong access