Conditional Strategy Stack Middleware.

gistfile1.php

<?php

class Conditional implements HttpKernelInterface {
    private $app;
    private $prefix;
    private $wrappedApp;

    public function __construct(HttpKernelInterface $app, $prefix, $spec) {
        $this->app = $app;
        $this->prefix = $prefix;
        $this->wrappedApp = $this->wrapApp($spec);
    }

    public function handle(Request $request, $type = MASTER, $catch = true) {
        $app = $this->requestMatchesPrefix($request) ? $this->wrappedApp : $this->app;
        return $app->handle($request, $type, $catch);
    }

    private function wrapApp($spec) {
        if (is_object($spec) && method_exists($spec, '__invoke')) {
            return $spec($this->app);
        }

        list($className, $args) = $this->classSpec($spec);
        return (new ReflectionClass($className))
            ->newInstanceArgs($args);
    }

    private function classSpec($spec) {
        return is_string($spec) ? $this->stringSpec($spec) : $this->arraySpec($spec);
    }

    private function stringSpec($spec) {
        $className = $spec;
        $args = [];

        return [$className, $args];
    }

    private function arraySpec($spec) {
        $args = $spec;
        $className = array_shift($args);
        $args = array_merge([$this->app], $args);

        return [$className, $args];
    }

    private function requestMatchesPrefix(Request $request) {
        return 0 === strpos($request->getPath(), $this->prefix);
    }
}

$app = (new Stack\Builder())
    ->push('Stack\Conditional', '/foo', function ($app) { return new Ducks\Stack\GeoIp($app); })
    ->push('Stack\Conditional', '/foo', 'Ducks\Stack\GeoIp')
    ->push('Stack\Conditional', '/foo', ['Ducks\Stack\GeoIp', ['adapter' => '...']])
    ->resolve($app);