Как применить паттерн Singltone к классу?

Мне нужно применить паттерн Singltone к двум классом (FileBox и DbBox), но т.к. я только учусь, не могу справится с этой задачей.

class FileBox extends BoxAbstract
{
    private $file;

    public function __construct($file)
    {
        $this->file = $file;
    }

    public function save()
    {
        file_put_contents($this->file, serialize($this->data));
    }

    public function load()
    {
        $this->data = unserialize(file_get_contents($this->file));
    }
}

class DbBox extends BoxAbstract
{
    private $pdo;

    public function __construct(\PDO $pdo)
    {
        $this->pdo = $pdo;
    }

    public function save()
    {
        $this->pdo->beginTransaction();

        $this->pdo->query('DELETE FROM box')->execute();

        $stmt = $this->pdo->prepare("UPDATE INTO box (key, value) VALUES (:key, :value)");

        foreach ($this->data as $key => $value) {
            $stmt->bindValue(':key', $key);
            $stmt->bindValue(':value', serialize($value));
            $stmt->execute();
        }
        $this->pdo->commit();
    }

    public function load()
    {
        $stmt = $this->pdo->query('SELECT key, value FROM box_items');

        $data = [];

        while ($row = $stmt->fetch(\PDO::FETCH_ASSOC)) {
            $data[$row['key']] = unserialize($row['value']);
        }

        $this->data = $data;
    }

Вот что получилось:

class FileBox extends BoxAbstract
{
    private $file;
    private static $_instance;

    private function __construct($file)
    {
        $this->file = $file;
    }

    public static function init(): FileBox
    {
        if(self::$_instance === null){
            self::$_instance = new self();
        }
        return self::$_instance;            
    }

    public function save()
    {
        file_put_contents($this->file, serialize($this->data));
    }

    public function load()
    {
        $this->data = unserialize(file_get_contents($this->file));
    }
}

class DbBox extends BoxAbstract
{
    private $pdo;
    private static $_instance;

    private function __construct(\PDO $pdo)
    {
        $this->pdo = $pdo;
    }

    public static function init(): DbBox
    {
        if(self::$_instance === null){
            self::$_instance = new self();
        }
        return self::$_instance;            
    }

    public function save()
    {
        $this->pdo->beginTransaction();

        $this->pdo->query('DELETE FROM box')->execute();

        $stmt = $this->pdo->prepare("UPDATE INTO box (key, value) VALUES (:key, :value)");

        foreach ($this->data as $key => $value) {
            $stmt->bindValue(':key', $key);
            $stmt->bindValue(':value', serialize($value));
            $stmt->execute();
        }
        $this->pdo->commit();
    }

    public function load()
    {
        $stmt = $this->pdo->query('SELECT key, value FROM box_items');

        $data = [];

        while ($row = $stmt->fetch(\PDO::FETCH_ASSOC)) {
            $data[$row['key']] = unserialize($row['value']);
        }

        $this->data = $data;
    }