Object instantiation with new in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the new keyword do in PHP?

The <code>new</code> keyword creates a new instance (object) of a class. It tells PHP to build a fresh object based on the class blueprint.

beginner

How do you create an object from a class named <code>Car</code>?

You write $myCar = new Car();. This makes a new Car object and stores it in the variable $myCar.

beginner

What is the difference between a class and an object?

A class is like a blueprint or recipe. An object is the actual thing made from that blueprint. Using <code>new</code> turns the blueprint into a real object you can use.

beginner

Can you instantiate a class without using <code>new</code> in PHP?

No, in PHP you must use <code>new</code> to create an object from a class. Without <code>new</code>, you only have the class definition, not an object.

intermediate

What happens if you call <code>new</code> on a class with a constructor that requires parameters?

You must provide the required parameters inside the parentheses. For example, new Car('red', 2020);. Otherwise, PHP will give an error.

What does $obj = new MyClass(); do in PHP?

A Creates a new object of MyClass

B Deletes the MyClass definition

C Calls a function named MyClass

D Assigns a string 'MyClass' to $obj

Using new creates a new object from the class named MyClass.

Which keyword is used to create an object from a class in PHP?

A create

B object

C new

D make

The new keyword is used to instantiate (create) objects from classes.

What will happen if you forget parentheses when instantiating a class like new Car;?

A It will call the class as a function

B It will cause a syntax error

C It will create a string instead

D It will create the object anyway

In PHP, parentheses are optional if the constructor has no parameters, so new Car; works.

If a class constructor requires parameters, what must you do when using new?

A Call the constructor separately after new

B Pass the required parameters inside parentheses

C No parameters are needed

D Use a different keyword than new

You must provide the required parameters inside the parentheses when calling new.

What is stored in the variable after $obj = new ClassName();?

A An object instance of ClassName

B The class code

C A string 'ClassName'

D Nothing, it's empty

The variable holds the new object created from the class.

Explain in your own words what happens when you use the new keyword in PHP.

Think about turning a recipe into a cake.

Describe how you would create an object from a class that has a constructor with parameters.

Remember how you give ingredients when baking.

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