Singleton pattern in PHP - Mini Project: Build & Apply

Singleton pattern

📖 Scenario: You are building a PHP application where you need to ensure that only one instance of a class is created. This is useful when managing a single connection or configuration throughout your app.

🎯 Goal: Create a Singleton class in PHP that allows only one instance to be created and provides a method to get that instance.

📋 What You'll Learn

Create a class called Singleton

Add a private static variable called $instance to hold the single instance

Add a private constructor to prevent creating new instances from outside

Add a public static method called getInstance() that returns the single instance

Add a public method called showMessage() that prints a message to confirm the instance

💡 Why This Matters

🌍 Real World

Singleton pattern is used to manage shared resources like database connections or configuration settings where only one instance should exist.

💼 Career

Understanding Singleton helps in designing efficient and safe applications, a common requirement in backend and system programming jobs.

1

Create the Singleton class with a private static variable

Create a class called Singleton with a private static variable called $instance initialized to null.

PHP

<?php

// Your code here

Hint

Use private static ?Singleton $instance = null; inside the class.

2

Add a private constructor to prevent outside instantiation

Inside the Singleton class, add a private constructor method __construct() with an empty body.

PHP

<?php

class Singleton {
    private static ?Singleton $instance = null;

    // Your code here
}

Hint

Define private function __construct() { } inside the class.

3

Add the public static getInstance() method

Inside the Singleton class, add a public static method called getInstance() that checks if self::$instance is null. If it is, create a new Singleton and assign it to self::$instance. Then return self::$instance.

PHP

<?php

class Singleton {
    private static ?Singleton $instance = null;

    private function __construct() {
        // private constructor to prevent direct creation
    }

    // Your code here
}

Hint

Use public static function getInstance() with a check for self::$instance === null.

4

Add a method to show a message and print the singleton instance message

Inside the Singleton class, add a public method called showMessage() that prints "This is the Singleton instance.". Then, outside the class, get the singleton instance by calling Singleton::getInstance() and call showMessage() on it.

PHP

<?php

class Singleton {
    private static ?Singleton $instance = null;

    private function __construct() {
        // private constructor to prevent direct creation
    }

    public static function getInstance(): Singleton {
        if (self::$instance === null) {
            self::$instance = new Singleton();
        }
        return self::$instance;
    }

    // Your code here
}

// Your code here

Hint

Define public function showMessage() that echoes the message. Then call it on the instance returned by Singleton::getInstance().

Practice

1. What is the main purpose of the Singleton pattern in PHP?

easy

A. To ensure only one instance of a class is created

B. To allow multiple instances of a class

C. To inherit properties from multiple classes

D. To create objects without a constructor

Solution

1. Step 1: Understand Singleton pattern goal

   The Singleton pattern is designed to restrict a class to a single instance.

2. Step 2: Compare options with Singleton purpose

   Only To ensure only one instance of a class is created correctly states this purpose; others describe different concepts.

3. Final Answer:

   To ensure only one instance of a class is created -> Option A

4. Quick Check:

   Singleton = single instance only [OK]

Hint: Singleton means one and only one instance [OK]

Common Mistakes:

• Thinking Singleton allows multiple instances
• Confusing Singleton with inheritance
• Believing Singleton removes constructors

2. Which of the following is the correct way to declare the static instance variable in a Singleton class in PHP?

easy

A. public static $instance;

B. private static $instance;

C. protected $instance;

D. static $instance;

Solution

1. Step 1: Identify visibility for Singleton instance

   The instance variable must be private static to restrict access and keep one shared instance.

2. Step 2: Check options for correct syntax and visibility

   private static $instance; uses private static, which is correct. Others either lack visibility or static keyword.

3. Final Answer:

   private static $instance; -> Option B

4. Quick Check:

   Singleton instance is private static [OK]

Hint: Singleton instance must be private and static [OK]

Common Mistakes:

• Using public instead of private
• Omitting static keyword
• Using protected instead of private

3. What will be the output of the following PHP code?

class Singleton {
    private static ?Singleton $instance = null;
    public int $value;
    private function __construct() { $this->value = 10; }
    public static function getInstance(): Singleton {
        if (self::$instance === null) {
            self::$instance = new Singleton();
        }
        return self::$instance;
    }
}

$obj1 = Singleton::getInstance();
$obj2 = Singleton::getInstance();
$obj2->value = 20;
echo $obj1->value;

medium

A. 10

B. null

C. 20

D. Error

Solution

1. Step 1: Understand Singleton instance sharing

   Both $obj1 and $obj2 refer to the same Singleton instance.

2. Step 2: Trace value changes

   Changing $obj2->value to 20 affects $obj1->value because they are the same object.

3. Final Answer:

   20 -> Option C

4. Quick Check:

   Singleton shares one object, so value = 20 [OK]

Hint: Singleton returns same object, changes reflect everywhere [OK]

Common Mistakes:

• Assuming separate objects for each getInstance call
• Expecting original value 10 after change
• Thinking code causes error

4. Identify the error in this Singleton implementation:

class Singleton {
    private static $instance;
    private function __construct() {}
    public static function getInstance() {
        if (!isset(self::$instance)) {
            return new Singleton();
        }
        return self::$instance;
    }
}

medium

A. No error, code is correct

B. Constructor should be public

C. Static variable should be public

D. getInstance() does not assign the new object to static instance

Solution

1. Step 1: Check getInstance method logic

   The method creates a new Singleton but does not save it to self::$instance.

2. Step 2: Identify missing assignment

   Without assigning, every call returns a new object, breaking Singleton pattern.

3. Final Answer:

   getInstance() does not assign the new object to static instance -> Option D

4. Quick Check:

   Missing assignment breaks Singleton [OK]

Hint: Always assign new instance to static variable in getInstance [OK]

Common Mistakes:

• Making constructor public
• Not assigning instance in getInstance
• Changing static variable visibility incorrectly

5. You want to extend a Singleton class in PHP but ensure the child class also follows Singleton rules independently. Which approach is best?

hard

A. Use separate static instance variables in parent and child classes

B. Share the parent's static instance variable in the child class

C. Make constructor public in child class

D. Avoid inheritance and duplicate Singleton code

Solution

1. Step 1: Understand Singleton inheritance challenge

   Parent and child classes must maintain separate single instances to avoid conflicts.

2. Step 2: Use separate static variables

   Each class having its own static instance variable ensures independent Singleton behavior.

3. Final Answer:

   Use separate static instance variables in parent and child classes -> Option A

4. Quick Check:

   Separate static instances keep Singleton independent [OK]

Hint: Each class needs its own static instance variable [OK]

Common Mistakes:

• Sharing static instance causes one instance for both classes
• Making constructor public breaks Singleton
• Avoiding inheritance unnecessarily