Constructor promotion in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class using constructor promotion

What is the output of this PHP code using constructor promotion?

PHP

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

A Alice is years old.

B Alice is 30 years old.

C Error: Property age is not defined

D 30 is Alice years old.

💡 Hint

Constructor promotion automatically creates and assigns properties.

The constructor promotion syntax creates public properties $name and $age and assigns the passed values. So accessing $user->name and $user->age works as expected.

❓ Predict Output

intermediate

Output when mixing promoted and non-promoted properties

What will this PHP code output?

PHP

<?php
class Product {
    public string $category;
    public function __construct(
        public string $name,
        public float $price
    ) {
        $this->category = "General";
    }
}
$p = new Product("Book", 12.5);
echo $p->name . ' costs $' . $p->price . ' in category ' . $p->category;

A Book costs $12.5 in category General

B Book costs $ in category General

C Error: Property category is not initialized

D 12.5 costs $Book in category General

💡 Hint

Non-promoted properties must be initialized manually.

The property $category is declared but not promoted, so it is initialized inside the constructor. The promoted properties $name and $price are automatically assigned.

🔧 Debug

advanced

Identify the error in constructor promotion usage

What error does this PHP code produce?

PHP

<?php
class Car {
    public function __construct(
        private string $make,
        protected int $year = 2020,
        public $model
    ) {}
}
$car = new Car("Toyota", 2018, "Corolla");
echo $car->make;

A Notice: Undefined property: Car::$make

B Parse error: Default value for parameter must be constant

C Fatal error: Cannot access private property Car::$make

D No error, outputs: Toyota

💡 Hint

Private properties cannot be accessed directly outside the class.

The property $make is private, so trying to echo $car->make outside the class causes a fatal error due to visibility.

📝 Syntax

advanced

Which option causes a syntax error in constructor promotion?

Which of these constructor declarations will cause a syntax error in PHP?

A public function __construct(public int $x = 5, private string $y) {}

B public function __construct(public int $x, private string $y = "default") {}

C public function __construct(public int $x, private string $y) {}

D public function __construct(public int $x = 5, private string $y = "default") {}

💡 Hint

In PHP, parameters with default values must come after parameters without defaults.

Option A has a parameter with a default value ($x) before a parameter without a default ($y), which is invalid syntax.

🚀 Application

expert

Count properties created by constructor promotion

How many properties does this PHP class have after construction?

PHP

<?php
class Employee {
    public string $department = "HR";
    public function __construct(
        public string $name,
        private int $id,
        protected float $salary = 50000.0
    ) {}
}
$emp = new Employee("John", 1234);

A 3

B 5

C 2

D 4

💡 Hint

Count all declared properties including promoted and non-promoted.

The class has one non-promoted property $department and three promoted properties: $name, $id, $salary. Total is 4 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