This document discusses PHP dependency injection. It defines dependency injection as components being given their dependencies through constructors, methods, or directly into fields. Constructor injection is best for required dependencies, while setter injection is best for optional dependencies. Modern PHP frameworks heavily use dependency injection to provide decoupled yet cohesive components. Examples are given of constructor, setter, and property injection in PHP code. The benefits of dependency injection in loosening coupling between classes is demonstrated. Best practices for PHP dependency injection are outlined. Resources for learning more about PHP dependency injection are provided.

1. PHP DEPENDENCY
INJECTION
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@denis_ristic

2. PHP DEPENDENCY INJECTION
INTRO
‣ Dependency Injection is where components are given their
dependencies through their constructors, methods, or directly into
fields
‣ As a rule of thumb, constructor injection is best for required
dependencies, like in our example, and setter injection is best for
optional dependencies, like a cache object for instance.
‣ Nowadays, most modern PHP frameworks heavily use Dependency
Injection to provide a set of decoupled but cohesive components
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3. PHP DEPENDENCY INJECTION
CONSTRUCTOR, SETTER & PROPERTY INJECTION
<?php
class User
{
function __construct($storage)
{
$this->storage = $storage;
}
// ...
}
class User
{
function setSessionStorage($storage)
{
$this->storage = $storage;
}
// ...
}
class User
{
public $sessionStorage;
}
$user->sessionStorage = $storage;
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4. PHP DEPENDENCY INJECTION
EXAMPLE OF USAGE
<?php
// symfony: A constructor injection example
$dispatcher = new sfEventDispatcher();
$storage = new sfMySQLSessionStorage(array('database' => 'session', 'db_table' => 'session'));
$user = new sfUser($dispatcher, $storage, array('default_culture' => 'en'));
// Zend Framework: A setter injection example
$transport = new Zend_Mail_Transport_Smtp('smtp.gmail.com', array(
'auth' => 'login',
'username' => 'foo',
'password' => 'bar',
'ssl' => 'ssl',
‘port' => 465,
));
$mailer = new Zend_Mail();
$mailer->setDefaultTransport($transport);
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5. PHPUNIT
EXAMPLE CLASS WITHOUT & WITH DI
<?php
class StoreService
{
public function getStoreCoordinates($store) {
$geolocationService = new GoogleMaps();
// or $geolocationService =
GoogleMaps::getInstance() if you use singletons
return $geolocationService-
>getCoordinatesFromAddress($store->getAddress());
}
}
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class StoreService {
private $geolocationService;
public function __construct(GeolocationService
$geolocationService) {
$this->geolocationService = $geolocationService;
}
public function getStoreCoordinates($store) {
return $this->geolocationService-
>getCoordinatesFromAddress($store->getAddress());
}
}
interface GeolocationService {
public function getCoordinatesFromAddress($address);
}
class GoogleMaps implements GeolocationService { ... }
class OpenStreetMap implements GeolocationService { ... }
// The StoreService is no longer tightly coupled to its dependency.

6. PHP DEPENDENCY INJECTION
BEST PRACTICES
‣ Here are some basic rules to follow:
‣ never get an entry from the container directly (always use dependency
injection)
‣ more generally, write code decoupled from the container
‣ type-hint against interfaces, configure which implementation to use in the
container's configuration
‣ Writing controllers - inject dependencies in properties
‣ Writing services - start using constructor injection and autowiring
‣ Using libraries - define dependencies in your configuration file. It is
recommended using an anonymous function when the configuration gets a bit
complex (anonymous function allows you to write real PHP code).
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7. PHP DEPENDENCY INJECTION
PHP DI RESOURCES
‣ PHP DI
‣ http://php-di.org/doc/getting-started.html
‣ What is dependency injection?
‣ http://fabien.potencier.org/what-is-dependency-injection.html
‣ Dependency Injection in PHP
‣ https://code.tutsplus.com/tutorials/dependency-injection-in-php--
net-28146
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