Singleton pattern in PHP - Step-by-Step Execution

Concept Flow - Singleton pattern

Start

↓

Check if instance exists?

Yes→Return existing instance

No↓

Create new instance

↓

Store instance

↓

Return new instance

↓

End

The Singleton pattern checks if an instance exists; if not, it creates one, stores it, and returns it. Otherwise, it returns the stored instance.

Execution Sample

PHP

<?php
class Singleton {
  private static $instance = null;
  private function __construct() {}
  public static function getInstance() {
    if (self::$instance === null) {
      self::$instance = new Singleton();
    }
    return self::$instance;
  }
}
$one = Singleton::getInstance();
$two = Singleton::getInstance();
var_dump($one === $two); // true
?>

This code creates a Singleton class that only allows one instance and returns the same instance every time.

Execution Table

Step	Action	Condition	Result	Instance State	Output
1	Call getInstance()	self::$instance === null	True	null	Create new Singleton instance
2	Assign new Singleton to self::$instance	-	Instance stored	Singleton object	-
3	Return self::$instance	-	Return stored instance	Singleton object	Singleton object
4	Call getInstance() again	self::$instance === null	False	Singleton object	Return existing instance
5	Return self::$instance	-	Return stored instance	Singleton object	Singleton object
6	Compare $one === $two	-	True	Singleton object	bool(true)
7	End	-	-	Singleton object	-

💡 Execution stops after returning the stored Singleton instance and confirming both variables point to the same object.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
self::$instance	null	Singleton object	Singleton object	Singleton object	Singleton object
$one	undefined	Singleton object	Singleton object	Singleton object	Singleton object
$two	undefined	undefined	undefined	Singleton object	Singleton object

Key Moments - 3 Insights

Why does getInstance() create the object only once?

Because it checks if self::$instance is null (see Step 1 and Step 4 in execution_table). If not null, it returns the existing instance instead of creating a new one.

Why are $one and $two exactly the same object?

Because getInstance() returns the same stored instance each time (see Steps 3 and 5). The comparison in Step 6 confirms they reference the same object.

Why is the constructor private?

To prevent creating new instances outside getInstance(), ensuring only one instance exists (not shown in table but essential for Singleton).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of self::$instance after Step 2?

A null

B undefined

C Singleton object

D bool(true)

💡 Hint

Check the 'Instance State' column at Step 2 in execution_table.

At which step does getInstance() return the existing instance instead of creating a new one?

A Step 1

B Step 4

C Step 3

D Step 6

💡 Hint

Look at the 'Condition' and 'Result' columns for Step 4 in execution_table.

If the constructor was public, what would change in the execution_table?

A Multiple instances could be created outside getInstance()

B self::$instance would be null after Step 2

C getInstance() would never create an instance

D The comparison in Step 6 would be false

💡 Hint

Think about how private constructor controls instance creation (see key_moments).

Concept Snapshot

Singleton pattern ensures a class has only one instance.
Use a private static variable to hold the instance.
Make the constructor private to prevent direct creation.
Provide a public static method to get the instance.
If instance exists, return it; else create and store it.
This pattern controls resource use and consistency.

Full Transcript

The Singleton pattern in PHP uses a private static variable to hold the single instance of a class. The constructor is private to prevent creating new instances directly. The public static method getInstance() checks if the instance exists; if not, it creates and stores it, then returns it. If it exists, it returns the stored instance. This ensures only one object of the class exists. Variables $one and $two both get the same instance, confirmed by comparing them. This pattern is useful when you want to control resource usage or ensure consistent state across your program.