Is there a use-case for singletons with database access in PHP?

Question

I access my MySQL database via PDO. I\'m setting up access to the database, and my first attempt was to use the following:

The first thing I thought of is glob

Answer 1

First, I just want to say that I don't find much uses to the Singleton pattern. Why would one want to keep a single object thorough the whole application? Especially for databases, what if I want to connect to another database server? I have to disconnect and reconnect every time...? Anyway...

There are several drawbacks to using globals in an application (which is what the traditional use of the Singleton pattern does):

• Difficult to unit test
• Dependency injection issues
• Can create locking issues (multi-threaded application)

Use static classes instead of a singleton instance provides some of the same drawbacks as well, because the biggest problem of singleton is the static getInstance method.

You can limit the number of instances a class can have without using the traditional getInstance method:

class Single {

    static private $_instance = false;

    public function __construct() {
        if (self::$_instance)
           throw new RuntimeException('An instance of '.__CLASS__.' already exists');

        self::$_instance = true;
    }

    private function __clone() {
        throw new RuntimeException('Cannot clone a singleton class');
    }

    public function __destruct() {
        self::$_instance = false;
    }

}

$a = new Single;
$b = new Single; // error
$b = clone($a); // error
unset($a);
$b = new Single; // works

This will help on the first the points mentioned above: unit testing and dependency injection; while still making sure a single instance of the class exist in your application. You could, per example, just pass the resulting object to your models (MVC pattern) for them to use.