Constructor method in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor method in PHP?

A constructor method is a special function inside a class that runs automatically when a new object is created. It usually sets up initial values or states for the object.

beginner

How do you name a constructor method in PHP 7 and later?

You name the constructor method __construct(). This is a magic method that PHP calls automatically when creating an object.

beginner

What happens if you do not define a constructor in a PHP class?

If no constructor is defined, PHP creates the object without running any setup code automatically. You can still set properties later, but no automatic initialization happens.

beginner

Show a simple example of a PHP class with a constructor that sets a property.

Example:<br><pre>&lt;?php
class Car {
  public $color;
  public function __construct($color) {
    $this->color = $color;
  }
}
$myCar = new Car('red');
echo $myCar->color; // Outputs: red
?&gt;</pre>

beginner

Why use a constructor method instead of setting properties after creating an object?

Using a constructor ensures the object is ready to use immediately after creation. It helps avoid forgetting to set important properties and keeps code cleaner and safer.

What is the correct name for a constructor method in PHP 7+?

A __construct()

B constructor()

C init()

D start()

The magic method __construct() is the standard constructor name in PHP 7 and later.

When is a constructor method called?

A When a method is called

B When an object is created

C When a property is accessed

D When the script ends

The constructor runs automatically right after an object is created.

What keyword is used inside a constructor to refer to the current object?

A $this

B $self

C $current

D $obj

In PHP, $this refers to the current object inside class methods.

What happens if you create an object from a class without a constructor?

A The object cannot be created

B PHP throws an error

C The object is created without automatic setup

D The object runs a default constructor

Without a constructor, the object is created but no automatic initialization code runs.

Which of these is a good reason to use a constructor?

A To call static methods

B To delay object creation

C To delete the object

D To set initial property values automatically

Constructors help set up initial values so the object is ready to use immediately.

Explain what a constructor method is and why it is useful in PHP classes.

Think about what happens right after you create a new object.

Write a simple PHP class with a constructor that sets a property called 'name'.

Use __construct and $this->name = ...

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