Singleton pattern in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Singleton class property.

PHP

<?php
class Singleton {
    private static [1];
}

Drag options to blanks, or click blank then click option'

A$unique

B$object

C$single

D$instance

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-static property for the instance.

Naming the property something other than $instance.

The Singleton class uses a private static property named $instance to hold the single instance.

2fill in blank

medium

Complete the code to prevent creating multiple instances by making the constructor private.

PHP

<?php
class Singleton {
    private function [1]() {}
}

Drag options to blanks, or click blank then click option'

A__clone

B__destruct

C__construct

D__invoke

Attempts:

3 left

💡 Hint

Common Mistakes

Making the constructor public or protected.

Confusing constructor with clone or destructor.

The constructor __construct is made private to prevent creating new instances from outside the class.

3fill in blank

hard

Fix the error in the method that returns the single instance.

PHP

<?php
class Singleton {
    private static $instance;
    private function __construct() {}
    public static function getInstance() {
        if (self::[1] === null) {
            self::$instance = new Singleton();
        }
        return self::$instance;
    }
}

Drag options to blanks, or click blank then click option'

A$instance

Binstance

C$Instance

DInstance

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the dollar sign when accessing static property.

Using wrong case for the property name.

The static property must be accessed as self::$instance with the dollar sign.

4fill in blank

hard

Fill both blanks to prevent cloning and unserializing the Singleton instance.

PHP

<?php
class Singleton {
    private function [1]() {}
    public function [2]() {
        throw new \Exception("Cannot clone singleton");
    }
}

Drag options to blanks, or click blank then click option'

A__wakeup

B__clone

C__sleep

D__destruct

Attempts:

3 left

💡 Hint

Common Mistakes

Not preventing cloning or unserializing.

Using wrong magic method names.

The __wakeup method is made private to prevent unserializing, and __clone is made public but throws an exception to prevent cloning.

5fill in blank

hard

Fill all three blanks to complete the Singleton class with instance property, constructor, and getInstance method.

PHP

<?php
class Singleton {
    private static [1];
    private function [2]() {}
    public static function [3]() {
        if (self::$instance === null) {
            self::$instance = new Singleton();
        }
        return self::$instance;
    }
}

Drag options to blanks, or click blank then click option'

A$instance

B__construct

CgetInstance

D__clone

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong names for property or methods.

Making constructor public.

The Singleton class has a private static property $instance, a private constructor __construct, and a public static method getInstance to return the single instance.

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