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<?php interface Router { public function serve($uri); } class FlatRouter implements Router { public function serve($uri) { // send()s instance of HTTP\Request, no return value echo 'Serving request: ', $uri, ' using ', __CLASS__; } } class NestedRouter implements Router { public function serve($uri) { // send()s instance of HTTP\Request, no return value echo 'Serving request: ', $uri, ' using ', __CLASS__; } } abstract class Application { protected static $instances = array(); private $properties = array(); protected static function getInstance() { return isset(self::$instances[$className = get_called_class()]) ? self::$instances[$className] : self::$instances[$className] = new static(); } protected function __construct() { } public function __set($propertyName, $value) { $this->properties[$propertyName] = $value; } public function __get($propertyName) { return is_callable($this->properties[$propertyName]) ? $this->properties[$propertyName] = $this->properties[$propertyName]($this) : $this->properties[$propertyName]; } } abstract class WebApplication extends Application { protected $routerClassName = 'FlatRouter'; protected function __construct() { $this->router = function ($application) { $routerClassName = $application->routerClassName; return new $routerClassName(); }; } public static function serve($uri) { $instance = static::getInstance(); $instance->router->serve($uri); } } final class FooBarWebApplication extends WebApplication { protected $routerClassName = 'NestedRouter'; } FooBarWebApplication::serve('http://foo.bar/path/to/resource');