Singleton pattern in PHP - Time & Space Complexity

Time Complexity: Singleton pattern

O(1)

Understanding Time Complexity

Let's explore how the time cost grows when using the Singleton pattern in PHP.

We want to know how the program's steps increase as we create or access the Singleton instance more times.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

    private function __construct() {}

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

$single = Singleton::getInstance();

This code creates a Singleton class that only allows one instance to be created and returns it on request.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking if the instance exists and returning it.
• How many times: Each time getInstance() is called.

How Execution Grows With Input

Each call to getInstance() does a quick check and returns the stored instance.

Input Size (n)	Approx. Operations
10	10 simple checks and returns
100	100 simple checks and returns
1000	1000 simple checks and returns

Pattern observation: The work grows directly with the number of calls, but each call is very fast.

Final Time Complexity

Time Complexity: O(1)

This means the time to get the Singleton instance is constant for each call after the first creation.

Common Mistake

[X] Wrong: "Creating the Singleton instance is slow every time I call getInstance()."

[OK] Correct: The instance is created only once; later calls just return the stored object quickly.

Interview Connect

Understanding how Singleton controls instance creation helps you explain resource management in real projects.

Self-Check

"What if the Singleton created a new instance every time instead of storing one? How would the time complexity change?"

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