PHP Singletons, Sub-Classing, and HAS-A Relationships

I’ve been very busy these last fews weeks and have neglected making any posts. While there are a variety of subjects I’d love to post about, they’ll most likely have to wait until the next year. However, I thought I might be able to throw up a quick example of how to use PHP Singletons and extending public classes.

The Singleton Pattern “is a design pattern that is used to restrict instantiation of a class to one object.” (Wikipedia) In PHP, a singleton is created by making the constructor protected or private. “How do you create an object then?” Its done in two parts. First, you make a static class variable that is private, to hold the instance. Second, create a static function to return the instance. Here is an example:

<?php

// My Singleton Class
class MySingleton
{
	// Singleton instance
	static private $_instance = null;

	// Class Variables
	protected $name;

	protected function __construct()
	{
		$this->name = 'John';
	}

	static public function GetInstance()
	{
		// Check to see if the instance is null
		if(!self::$_instance)
		{
			self::$_instance = new MySingleton();
		}

		return self::$_instance;
	}

	public function GetName()
	{
		return $this->name;
	}
}

// Use the Singleton Class
$class = MySingleton::GetInstance();
echo "Hello, my name is: ".$class->GetName();

// Or you can call the function this way:
echo "Hello, my name is: ".MySingleton::GetInstance()->GetName();
?>

Singletons can be create if you need only one instance of a class. You must be careful, however, because you don’t want to “lock” yourself into an implementation where you really need more than one instance.

Now, what if I want to have a singleton of a current class? Let me give you an example. I have a project with a User class. Its used all over the website for display user’s information. Now, I wanted to create a class called CurrentUser, which is a singleton that holds the instance for the currently logged in user. Why would I want to do this? Once I get a user’s information, I only want to have to retrieve it once. I was running into problems where different parts of the website would request the current user’s information over, and over.

So at first I tried to sub-class the User class. Something like this:

<?php

class User
{
	public function __construct()
	{
		// .. Construction code ..
	}
}

class CurrentUser extends User
{
	static private $_instance = null;

	// .. insert code her ..
	private function __construct()
	{
		// .. code here ..
	}

	static public function GetInstance()
	{
		if(!self::$_instance)
		{
			self::$_instance = new User();
		}

		return self::$_instance;
	}
}

$user = CurrentUser::GetInstance();

?>

There is only one problem: when I tried to declare the constructor private, it threw an error:

Fatal error: Access level to CurrentUser::__construct() must be public (as in class User) in /path/to/file.php on line 73

The problem is you can’t protect or privatize a public function. If a parent has a less restrictive function, it’s child cannot make it more restrictive. But with a singleton, the pattern’s goal is to restrict the construction of an object to ensure only one instance. So how do we overcome this problem? HAS-A relationship, instead of a IS-A.

HAS-A relationship uses class members instead inheritance to accomplish it’s design. Whats the difference? Instead of CurrentUser “being” a sub class of User, it “has” a member variable containing an instance of a User object. We then don’t have to worry about overriding the constructor. Here is how you could do it:

<?php

class User
{
	public function __construct()
	{
		// .. Construction code ..
	}
}

class CurrentUser
{
	// Singleton Instance
	static private $_instance = null;

	// This class HAS-A user instance
	private $user = null;

	// .. insert code her ..
	private function __construct()
	{
		// .. constructor code ..
		$this->user = new User();

	}

	static public function GetInstance()
	{
		if(!self::$_instance)
		{
			self::$_instance = new CurrentUser();
		}

		// Instead of returning the singleton instance, return it's user instance
		return self::$_instance->user;
	}
}

$user = CurrentUser::GetInstance();

?>

Nifty how that works. As developers, its easy to think “sub-classing” is the best method. It feels very “OOP” and it works in many ways. However, many times our design in the long run is much more complicated than sub-classing can handle. HAS-A relationships can offer more flexibility in the long run. Hopefully this can help out some people, and give others an idea on how to make Singletons work for you.

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