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Creating a Singleton Class

Basic Implementation​

To create a singleton class, use the Singleton trait and ensure your constructor is private or protected:

<?php
class Example
{
    // Use the Singleton trait to implement the pattern
    use \ByJG\DesignPattern\Singleton;

    // You can add properties to your singleton
    public string $someProperty;

    // The constructor MUST be private or protected
    private function __construct()
    {
        // Optional initialization code
        $this->someProperty = "Initial value";
    }

    // Add your own methods and properties here
    public function doSomething(): void
    {
        // Your code here
    }
}

Important Rules​

Constructor Visibility

Your class MUST use a private or protected constructor. If the constructor is public, the singleton pattern will be broken as users could create multiple instances directly.

No Constructor Arguments

Singleton classes do not accept arguments in the constructor. Use setter methods or configure the instance after retrieval if you need to customize it.

Protected Operations

Attempting to clone, serialize, or unserialize a singleton will throw a SingletonException.

Real-World Example​

<?php
class DatabaseConnection
{
    use \ByJG\DesignPattern\Singleton;

    private $connection;

    private function __construct()
    {
        // Establish database connection once
        $this->connection = new PDO(
            'mysql:host=localhost;dbname=mydb',
            'user',
            'password'
        );
    }

    public function query(string $sql): array
    {
        return $this->connection->query($sql)->fetchAll();
    }
}

What You Get​

When you use the Singleton trait, you automatically get:

  • getInstance() - Static method to retrieve the single instance
  • Clone protection - __clone() throws SingletonException
  • Serialization protection - __sleep() throws SingletonException
  • Unserialization protection - __wakeup() throws SingletonException

Next Steps​