While Singletons have become a Pattern-Non-Grata over the years, you still find it surprisingly often in PHP applications and frameworks. This talk will explain what the Singleton pattern is, how it works in PHP and why you should avoid it in your application.

1. Singletons in
Why they are bad and how you can eliminate
them from your applications

2. Gordon Oheim
@go_oh

4. a.k.a.
The GoF Book

5. Singleton

6. Creational Pattern
„control when and how objects are
created“

7. “Ensure a class has only one
instance, and provide a global
access point to it.”

8. Singleton
Singleton
- instance : Singleton
- instance : Singleton
+
+ getInstance()
getInstance() :
: Singleton
Singleton
-
- __construct()
__construct() :
: void
void
-
- __clone()
__clone() :
: void
void
-
- __wakeup()
__wakeup() :
: void
void

9. class Singleton
…
public static function getInstance()
{
if (!isset(self::$instance)) {
self::$instance = new self;
}
return self::$instance;
}
}

10. abstract class Singleton
…
public static function getInstance()
{
return isset(static::$instance)
? static::$instance
: static::$instance = new static;
}
final private function __construct()
{
static::init();
}
protected function init() {}
}
class A extends Singleton {
protected static $instance;
}

11. trait Singleton
…
protected static $instance;
final public static function getInstance()
{
return isset(static::$instance)
? static::$instance
: static::$instance = new static;
}
final private function __construct() {
static::init();
}
protected function init() {}
}
class A {
use Singleton;
}

12. „Nobody should need a mechanism to make
it as easy as pie to clutter the code base
with singletons“
- Sebastian Bergmann

13. Testing Singletons

14. „In my stilted view of the universe anything that
impedes testing is something to be avoided. There
are those who don't agree with this view, but I'll
pit my defect rates against theirs any time.“
- Robert „Uncle Bob“ C. Martin

15. class TableDataGateway
…
public function __construct()
{
$this->db = Db::getInstance();
}
public insert(array $data)
{
$this->db->sql('INSERT …', $data);
}
}

16. class TableDataGatewayTest extends PHPUnit…
/**
* @dataProvider provideInsertData
*/
public function testInsertCallsDbWithData($data)
{
$gateway = new TableDataGateway;
$gateway->insert($data);
$this->assertSame(
$data,
$gateway->findById($data['id'])
);
}
}

17. throws LogicException:
No key found for 'pdo' in Registry

18. class Db extends Singleton
…
protected function init()
{
$this->logger = Logger::getInstance();
$this->pdo = new PDO(Registry::get('pdo'));
}
public sql($query, $data)
{
$this->logger->log(printf(
'Executed %s with %s',
$query,
implode(',', $data)
));
$sth = $this->pdo->prepare($query);
return $sth->execute($data);
}
}

19. Singletons are pathological liars
- Miško Hevery

20. class TableDataGatewayTest extends PHPUnit…
/**
* @dataProvider provideInsertData
*/
public function testInsertCallsDbWithData($data)
{
Registry::set('pdo', array('…'));
Registry::set('log', '/dev/null');
$gateway = new TableDataGateway;
$gateway->insert($data);
$this->assertSame(
$data,
$gateway->findById($data['id'])
);
}
}

21. Your tests are not isolated.
You still need a real database.
No easy way to mock the Db Singleton.

22. „The real problem with Singletons is that they
give you such a good excuse not to think carefully
about the appropriate visibility of an object.“
- Kent Beck

23. Hard to change code
manifests itself in a cascade of
subsequent changes in dependent
code

24. Fragile code
breaks in many places when you
change just one place

25. Non-reusable code
is code that you cannot reuse in
another project because it contains too
much extra baggage

26. S.O.L.I.D.

27. Single Responsibility Principle
A class should have one, and only one,
reason to change.

28. Open Closed Principle
You should be able to extend a classes
behavior, without modifying it.

29. Liskov Substitution Principle
Derived classes must be substitutable
for their base classes.

30. Interface Segregation Principle
Make fine grained interfaces that are client
specific.

31. Dependency Inversion Principle
Depend on abstractions, not on concretions.

32. Solid Singleton?

33. SRP Violation
Creation Logic + Business Logic
= 2 reasons to change
Mitigated by using Abstract Singletons
But responsibilities are still strongly
coupled through inheritance

34. OCP Violation
In PHP < 5.2 Singletons are closed for extending
Boilerplate code has to be removed when no
longer needed

35. DIP Violation
Global access breaks encapsulation
Hides dependencies
Adds concrete dependencies into Clients
Signature lies

36. Singletons in the Wild

37. Config FooController
Response
FrontController
ThatService
Request Cache
Database
Application BarController
Registry Logger
ThisService
Bootstrap Acl

38. Config FooController
Response
FrontController
ThatService
Request Cache
Database
Application BarController
Registry Logger
ThisService
Bootstrap Acl

39. Recap:
“Ensure a class has only one
instance, and provide a global
access point to it.”

40. „So much of the Singleton pattern is about
coaxing language protection mechanisms into
protecting this one aspect: singularity. I guess it
is important, but it seems to have grown out of
proportion.“
- Ward Cunningham

41. You do not need to ensure singularity
when you are going to instantiate the
object only once anyway.

42. You do not need to provide a Global
Access Point when you can inject the
object.

43. class TableDataGateway
…
public function __construct()
{
$this->db = Db::getInstance();
}
}
class Db extends Singleton
…
protected function init()
{
$this->logger = Logger::getInstance();
$this->pdo = new PDO(Registry::get('pdo'));
}
}

44. class TableDataGateway
…
public function __construct(Db $db)
{
$this->db = $db;
}
}
class Db
…
public function __construct(PDO $pdo, Log $logger)
{
$this->logger = $logger;
$this->pdo = $pdo;
}
}

45. But then I have to push dependencies all
the way through my object graph?!

46. Recap:
Creational Pattern
„control when and how objects are
created“

47. Use Builders and Factories to
create Collaborator Graphs

48. class UserControllerBuilder
{
public function build($config)
{
$log = new Log($config['log']);
$pdo = new PDO($config['…']);
$db = new Db($pdo, $log);
$tdg = new TableDataGateway($db);
return new UserController($tdg);
}
}

49. class UserControllerBuilder
{
public function build($config)
{
$log = new Log($config['log']);
$pdo = new PDO($config['…']);
??? $db = new Db($pdo, $log);
$tdg = new TableDataGateway($db);
return new UserController($tdg);
}
}

50. class UserControllerBuilder
{
public function build($config)
{
$db = new LogDecorator(
new PDO($config['…']);
new Log($config['log']);
);
$tdg = new TableDataGateway($db);
return new UserController($tdg);
}
}

51. // index.php
include '/path/to/autoload.php';
$config = new Config('/path/to/config.ini');
$router = new Router(
array(
'/user/{id}' => function() use ($config)
{
$builder = new UserControllerBuilder;
return $builder->build($config);
}
)
);
$router->route(
new Request($_GET, $_POST, $_SERVER),
new Response
);

52. „Singletons aren't necessary
when you can design or redesign
to avoid using them.“
- Joshua Kerievsky

53. „I'm in favor of dropping Singleton.
Its use is almost always a design
smell“
- Erich Gamma