Constructor method in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP constructor example?

Consider the following PHP class with a constructor. What will be printed when the code runs?

PHP

<?php
class Car {
    public $model;
    public function __construct($model) {
        $this->model = $model;
        echo "Car model is: " . $this->model . "\n";
    }
}
$car = new Car("Toyota");
?>

A No output

B Car model is:

C Fatal error: Constructor method not found

D Car model is: Toyota

💡 Hint

Look at what the constructor does when the object is created.

The constructor __construct is called automatically when a new object is created. It prints the model name passed during object creation.

❓ Predict Output

intermediate

What will be the value of $obj->name after this code runs?

Look at this PHP class with a constructor. What is the value of the property name after creating the object?

PHP

<?php
class Person {
    public $name;
    public function __construct($name) {
        $this->name = $name;
    }
}
$obj = new Person("Alice");
// What is $obj->name?
?>

A "Alice"

B "name"

C Undefined property error

D null

💡 Hint

The constructor sets the property from the argument.

The constructor assigns the passed argument "Alice" to the property name. So $obj->name is "Alice".

🔧 Debug

advanced

What error does this PHP constructor code produce?

This PHP class tries to use a constructor but has a mistake. What error will it cause?

PHP

<?php
class Animal {
    public $type;
    public function Animal($type) {
        $this->type = $type;
    }
}
$dog = new Animal("Dog");
echo $dog->type;
?>

A Outputs: Dog

B Notice: Undefined property: Animal::$type

C Fatal error: Call to undefined method Animal::__construct()

D Parse error: syntax error

💡 Hint

PHP 7+ uses __construct() as the constructor method, not the class name.

In modern PHP versions, the constructor must be named __construct(). Naming it the same as the class does not work and __construct() is missing, causing a fatal error.

🧠 Conceptual

advanced

Which statement about PHP constructors is true?

Choose the correct statement about constructor methods in PHP classes.

A Constructors cannot accept parameters in PHP.

B The constructor method __construct() is called automatically when an object is created.

C The constructor method must always be named after the class name.

D You must call the constructor manually after creating an object.

💡 Hint

Think about what happens when you create a new object.

In PHP, the __construct() method is called automatically when a new object is created. It can accept parameters and is the standard constructor method.

📝 Syntax

expert

Which option correctly defines a PHP constructor with a default parameter?

Select the PHP constructor method that correctly sets a default value for a parameter.

A public function __construct($name ?: "Guest") { $this->name = $name; }

B public function __construct($name) = "Guest" { $this->name = $name; }

C public function __construct($name = "Guest") { $this->name = $name; }

D public function __construct($name = Guest) { $this->name = $name; }

💡 Hint

Default parameters use = with quotes for strings.

Option C correctly uses = "Guest" to set a default string parameter. Option C and C have invalid syntax. Option C misses quotes around Guest.

Practice

1. What is the main purpose of a constructor method in a PHP class?

easy

A. To print the object details

B. To delete an object from memory

C. To automatically set up an object when it is created

D. To create a new class inside the existing class

Solution

1. Step 1: Understand what a constructor does

   A constructor is a special method that runs automatically when an object is created.

2. Step 2: Identify the purpose of this automatic setup

   This setup usually initializes properties or runs code needed for the object to work properly.

3. Final Answer:

   To automatically set up an object when it is created -> Option C

4. Quick Check:

   Constructor = automatic setup [OK]

Hint: Constructor runs automatically on object creation [OK]

Common Mistakes:

• Thinking constructor deletes objects
• Confusing constructor with printing methods
• Assuming constructor creates new classes

2. Which of the following is the correct way to declare a constructor method in PHP?

easy

A. function __construct() {}

B. function construct() {}

C. function _construct() {}

D. function __destruct() {}

Solution

1. Step 1: Recall the special constructor name in PHP

   PHP uses the exact name __construct with two underscores at the start.

2. Step 2: Check each option for correct syntax

   Only function __construct() {} uses the correct spelling and underscores for the constructor.

3. Final Answer:

   function __construct() {} -> Option A

4. Quick Check:

   Constructor name = __construct [OK]

Hint: Constructor always named __construct with two underscores [OK]

Common Mistakes:

• Missing one or both underscores
• Using 'construct' without underscores
• Confusing constructor with destructor (__destruct)

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

class Car {
  public $color;
  public function __construct($color) {
    $this->color = $color;
  }
  public function getColor() {
    return $this->color;
  }
}
$myCar = new Car('red');
echo $myCar->getColor();

medium

A. red

B. Car

C. null

D. Error

Solution

1. Step 1: Understand constructor parameter usage

   The constructor sets the property color to the value passed ('red').

2. Step 2: Check what getColor() returns

   getColor() returns the color property, which was set to 'red'.

3. Final Answer:

   red -> Option A

4. Quick Check:

   Constructor sets color = 'red' [OK]

Hint: Constructor sets properties used later [OK]

Common Mistakes:

• Forgetting to pass argument to constructor
• Expecting class name as output
• Assuming property is null without initialization

4. Identify the error in this PHP class constructor code:

class Person {
  public $name;
  public function __construct {
    $this->name = 'Alice';
  }
}

medium

A. Constructor name should be Person

B. Wrong property name

C. Missing semicolon after assignment

D. Missing parentheses after __construct

Solution

1. Step 1: Check constructor syntax

   In PHP, the constructor method must have parentheses even if no parameters are used.

2. Step 2: Identify the missing parentheses

   The code misses () after __construct, causing a syntax error.

3. Final Answer:

   Missing parentheses after __construct -> Option D

4. Quick Check:

   Constructor needs () even if empty [OK]

Hint: Always add () after __construct even if empty [OK]

Common Mistakes:

• Omitting parentheses in method declaration
• Thinking constructor name must match class
• Forgetting semicolon after statements

5. You want to create a PHP class Book that stores a title and author. The constructor should accept both values and store them. Which code correctly implements this?

hard

A. class Book {
public $title, $author;
public function Book($title, $author) {
$this->title = $title;
$this->author = $author;
}
}

B. class Book {
public $title;
public $author;
public function __construct($title, $author) {
$this->title = $title;
$this->author = $author;
}
}

C. class Book {
public $title;
public $author;
public function __construct() {
$title = '';
$author = '';
}
}

D. class Book {
public $title;
public $author;
public function __construct($title) {
$this->title = $title;
}
}

Solution

1. Step 1: Check constructor name and parameters

   PHP uses __construct with two parameters $title and $author, matching the requirement.

2. Step 2: Verify property assignments

   The code assigns both $title and $author to object properties using $this correctly.

3. Final Answer:

   class Book { public $title; public $author; public function __construct($title, $author) { $this->title = $title; $this->author = $author; } } -> Option B

4. Quick Check:

   Constructor with all needed parameters and assignments [OK]

Hint: Use __construct with all needed parameters and assign to properties [OK]

Common Mistakes:

• Using old-style constructor named after class
• Not passing all required parameters
• Not assigning parameters to properties