Object instantiation with new in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code?

Consider this PHP code that creates a new object and accesses its property. What will it print?

PHP

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

A Buddy

B Dog

C Error: Undefined property

D NULL

💡 Hint

Look at how the constructor sets the property and what is echoed.

The constructor sets the name property to 'Buddy'. Accessing $dog->name prints 'Buddy'.

❓ Predict Output

intermediate

What will this PHP code output?

This code creates two objects from the same class. What will be printed?

PHP

<?php
class Counter {
    public static $count = 0;
    public function __construct() {
        self::$count++;
    }
}
$a = new Counter();
$b = new Counter();
echo Counter::$count;
?>

A Error: Cannot access static property

B 1

C 0

D 2

💡 Hint

Static properties are shared across all instances.

Each time a Counter object is created, the static property $count increments. Two objects means $count is 2.

🔧 Debug

advanced

What error does this PHP code produce?

This code tries to create an object but has a mistake. What error will it cause?

PHP

<?php
class Car {
    private $model;
    public function __construct($model) {
        $this->model = $model;
    }
}
$car = new Car();
echo $car->model;
?>

A Fatal error: Cannot access private property Car::$model

B Notice: Undefined property: model

C Fatal error: Missing argument 1 for Car::__construct()

D No error, outputs empty string

💡 Hint

Check the constructor parameters and how the object is created.

The constructor requires one argument, but none was given, causing a fatal error.

📝 Syntax

advanced

Which option will cause a syntax error in PHP?

Look at these ways to instantiate an object. Which one is NOT valid PHP syntax?

A $obj = new MyClass();

B $obj = new MyClass[];

C $obj = new MyClass;

D $obj = new MyClass(5);

💡 Hint

Check the use of brackets after the class name.

Using square brackets [] after the class name is invalid syntax for object instantiation.

🚀 Application

expert

How many objects are created and what is the output?

This code creates objects inside a loop and prints a property. How many objects are created and what is printed?

PHP

<?php
class Item {
    public $id;
    public function __construct($id) {
        $this->id = $id;
    }
}
$items = [];
for ($i = 1; $i <= 3; $i++) {
    $items[] = new Item($i);
}
foreach ($items as $item) {
    echo $item->id . " ";
}
?>

A 3 objects created; output: '1 2 3 '

B 1 object created; output: '3 '

C 3 objects created; output: '3 3 3 '

D Error: Cannot use object in array

💡 Hint

Each loop iteration creates a new object with a different id.

The loop runs 3 times, creating 3 objects with ids 1, 2, and 3. The foreach prints each id separated by space.

Practice

1.

What does the new keyword do in PHP?

easy

A. It deletes an existing object.

B. It defines a new class.

C. It calls a function inside a class.

D. It creates a new instance of a class (an object).

Solution

1. Step 1: Understand the role of new

   The new keyword is used to create an object from a class blueprint.

2. Step 2: Differentiate from other keywords

   It does not delete objects, call functions, or define classes; it only creates objects.

3. Final Answer:

   It creates a new instance of a class (an object). -> Option D

4. Quick Check:

   new creates objects [OK]

Hint: Remember: new = create object from class [OK]

Common Mistakes:

• Thinking new deletes objects
• Confusing new with function calls
• Believing new defines classes

2.

Which of the following is the correct syntax to create a new object of class Car?

class Car {
  // class body
}

// Create object here

easy

A. $car = new Car();

B. $car = Car();

C. new $car = Car();

D. $car = Car.new();

Solution

1. Step 1: Recall PHP object creation syntax

   In PHP, to create an object, use $variable = new ClassName();.

2. Step 2: Check each option

   $car = new Car(); matches the correct syntax. The other options use invalid syntax: new before variable assignment, omitting the new keyword, or using dot notation.

3. Final Answer:

   $car = new Car(); -> Option A

4. Quick Check:

   Use new ClassName() to create objects [OK]

Hint: Use $obj = new ClassName(); always [OK]

Common Mistakes:

• Omitting the new keyword
• Placing new after variable
• Using dot notation like other languages

3.

What will be the output of the following PHP code?

class Dog {
  public $name;
  public function __construct($name) {
    $this->name = $name;
  }
  public function bark() {
    return "Woof! I am " . $this->name;
  }
}

$dog = new Dog("Buddy");
echo $dog->bark();

medium

A. Woof! I am Buddy

B. Woof! I am $name

C. Error: Undefined property

D. Woof! I am

Solution

1. Step 1: Understand constructor usage

   The constructor sets the name property to "Buddy" when the object is created.

2. Step 2: Analyze the bark method output

   The bark() method returns "Woof! I am " concatenated with the name property, which is "Buddy".

3. Final Answer:

   Woof! I am Buddy -> Option A

4. Quick Check:

   Constructor sets name, bark returns it [OK]

Hint: Constructor sets data; methods use it [OK]

Common Mistakes:

• Ignoring constructor parameters
• Using variable name instead of property
• Expecting syntax error due to missing parentheses

4.

Identify the error in this PHP code snippet:

class Person {
  public $age;
  public function __construct($age) {
    $this->age = $age;
  }
}

$person = Person(25);
echo $person->age;

medium

A. Property age should be private.

B. Constructor should not have parameters.

C. Missing new keyword when creating the object.

D. Cannot echo object property directly.

Solution

1. Step 1: Check object instantiation syntax

   The object is created by calling Person(25) without new, which is invalid in PHP.

2. Step 2: Correct the instantiation

   Use $person = new Person(25); to create the object properly.

3. Final Answer:

   Missing new keyword when creating the object. -> Option C

4. Quick Check:

   Always use new to instantiate objects [OK]

Hint: Always use new before class name to create object [OK]

Common Mistakes:

• Forgetting new keyword
• Thinking constructor can't have parameters
• Confusing property visibility with errors

5.

Given this class:

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

// Create an array of books with titles and authors
$booksData = [
  ['1984', 'George Orwell'],
  ['Brave New World', 'Aldous Huxley'],
  ['Fahrenheit 451', 'Ray Bradbury']
];

// Fill this array with Book objects
$books = [];
foreach ($booksData as $data) {
  // Fill here
}

Which line correctly creates and adds a new Book object to $books inside the loop?

hard

A. $books[] = new Book($data[1], $data[0]);

B. $books[] = new Book($data[0], $data[1]);

C. $books[] = new Book($data);

D. $books[] = Book($data[0], $data[1]);

Solution

1. Step 1: Understand constructor parameters

   The constructor expects two parameters: title and author, in that order.

2. Step 2: Match data array to constructor

   $data[0] is title, $data[1] is author, so new Book($data[0], $data[1]) is correct.

3. Step 3: Add object to array

   Using $books[] = appends the new object to the array.

4. Final Answer:

   $books[] = new Book($data[0], $data[1]); -> Option B

5. Quick Check:

   Use new with correct params and append [OK]

Hint: Match constructor params order exactly when using new [OK]

Common Mistakes:

• Omitting new keyword
• Passing whole array instead of separate params
• Swapping title and author order