Object instantiation with new in PHP - Time & Space Complexity

Time Complexity: Object instantiation with new

O(n)

Understanding Time Complexity

When we create new objects in PHP using new, it takes some time to set up each object.

We want to know how the time needed grows as we create more objects.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Item {
    public function __construct() {
        // Some setup code
    }
}

$items = [];
for ($i = 0; $i < $n; $i++) {
    $items[] = new Item();
}
    

This code creates $n new objects of the Item class and stores them in an array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating a new Item object inside the loop.
• How many times: Exactly $n times, once per loop iteration.

How Execution Grows With Input

Each new object takes a similar amount of time to create, so total time grows as we add more objects.

Input Size (n)	Approx. Operations
10	10 object creations
100	100 object creations
1000	1000 object creations

Pattern observation: The time grows directly in proportion to the number of objects created.

Final Time Complexity

Time Complexity: O(n)

This means if you double the number of objects, the time to create them roughly doubles too.

Common Mistake

[X] Wrong: "Creating objects inside a loop is instant and does not affect performance."

[OK] Correct: Each object creation takes some time, so doing it many times adds up and affects total time.

Interview Connect

Understanding how object creation time grows helps you write efficient code and explain your reasoning clearly in interviews.

Self-Check

"What if the constructor of Item did some heavy work like reading a file? How would the time complexity change?"

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