Object instantiation with new in PHP - Step-by-Step Execution

Concept Flow - Object instantiation with new

Define Class

↓

Call new ClassName()

↓

Allocate Memory for Object

↓

Run Constructor (if any)

↓

Return Object Reference

↓

Assign to Variable

↓

Use Object

This flow shows how PHP creates a new object: it defines the class, calls new, allocates memory, runs the constructor, returns the object, and assigns it to a variable.

Execution Sample

PHP

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

This code creates a Car object with color 'red' and prints the color.

Execution Table

Step	Action	Evaluation	Result
1	Define class Car	Class Car defined	Class Car available
2	Call new Car('red')	Allocate memory for new object	Memory allocated
3	Run __construct('red')	$this->color = 'red'	Object property color set to 'red'
4	Return object reference	Object created	Reference to Car object returned
5	Assign to variable $myCar	$myCar = Car object	$myCar holds reference to Car object
6	echo $myCar->color	Access property color	Output: red

💡 Execution ends after printing the color property of the object.

Variable Tracker

Variable	Start	After Step 5	Final
$myCar	undefined	Car object with color='red'	Car object with color='red'

Key Moments - 3 Insights

Why do we use 'new' to create an object?

Using 'new' tells PHP to allocate memory and run the constructor to create a fresh object, as shown in steps 2 and 3 of the execution table.

What does the constructor __construct do here?

The constructor sets the object's color property to 'red' during instantiation, as seen in step 3 where $this->color is assigned.

Why do we assign the new object to a variable?

Assigning to $myCar stores the reference so we can use the object later, shown in step 5 where $myCar holds the object.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $myCar->color after step 5?

A undefined

B 'red'

C null

D 'blue'

💡 Hint

Check step 3 where the constructor sets color, and step 5 where $myCar is assigned.

At which step is memory allocated for the new object?

A Step 2

B Step 1

C Step 4

D Step 6

💡 Hint

Look at the action column describing memory allocation.

If we did not assign the new object to $myCar, what would happen?

A PHP would throw an error.

B The constructor would not run.

C The object would be created but immediately lost.

D The object would be stored in $myCar automatically.

💡 Hint

Refer to step 5 about assignment and variable tracking.

Concept Snapshot

Object instantiation in PHP:
Use 'new ClassName()' to create an object.
This allocates memory and runs __construct.
Assign the object to a variable to use it.
Access properties with -> operator.
Example: $obj = new ClassName();

Full Transcript

This example shows how PHP creates an object using the new keyword. First, the class Car is defined. Then, new Car('red') is called, which allocates memory for the object and runs the constructor to set the color property to 'red'. The object reference is returned and assigned to the variable $myCar. Finally, echo $myCar->color prints 'red'. This process ensures the object is properly created and ready to use.

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