PHP: Is implementing the database layer as a singleton acceptable? Code inside

Question

I know singletons are bad. But is it bad for this, too?

class DaoMySQL {

    private static $instance;
    private $PDO;

    private function __construct() {
        $this->PDO = new PDO('mysql:dbname='.MYSQL_DEFAULT_DATABASE.';host='.MYSQL_HOSTNAME, MYSQL_USERNAME, MYSQL_PASSWORD);
        $this->PDO->query('SET NAMES \'utf8\'');
    }

    /**
     * @return DaoMySQL
     */
    static public function singleton() {
        if (!isset(self::$instance)) {
            $c = __CLASS__;
            self::$instance = new $c();
        }
        return self::$instance;
    }

    /**
     * @return PDO
     */
    public function getPDO() {
        return $this->PDO;
    }

}

To use this, I do something like this. (This is from my Bean class which all data objects extends.)

public function delete() {
    $calledClassName = get_called_class();
    $query = "DELETE FROM `" . $calledClassName::table . "` WHERE `id` = $this->id";
    return DaoMySQL::singleton()->getPDO()->exec($query);
}

Answer 1

Many people are starting to use Dependency Injection containers to manage their objects instead of using singletons. Perhaps it's worth a look? Then all you need to ensure is that objects can access the container. You can fetch everything else from there.

Personally I use sfServiceContainer from Symfony Components. It's a stand-alone DI container and seems quite popular these days.

Update

You don't need to use a framework or a library. Fabien Potencier's articles on dependency injection should give you a good enough grasp of DI to implement your own. But why reinvent the wheel? Not using a good, existing library smells of NIH.

Note that there are many other DI libraries besides the sfServiceContainer that I use. Also note that sfServiceContainer is a completely stand-alone library. It does not need Symfony or any other framework. All it requires is plain old PHP.