Zend\Stratigility\Utils::getArity PHP Method

    public static function getArity($callable)
    {
        if (is_object($callable)) {
            foreach (['__invoke', 'handle'] as $method) {
                if (!method_exists($callable, $method)) {
                    continue;
                }
                $r = new ReflectionMethod($callable, $method);
                return $r->getNumberOfRequiredParameters();
            }
            return 0;
        }
        if (!is_callable($callable)) {
            return 0;
        }
        // Handle static methods passed in Class::method format by re-casting
        // as an array callable.
        if (is_string($callable) && preg_match('/^(?P<class>[^:]+)::(?P<method>.*)$/', $callable, $matches)) {
            $callable = [$matches['class'], $matches['method']];
        }
        if (is_array($callable)) {
            list($class, $method) = $callable;
            $r = new ReflectionMethod($class, $method);
            return $r->getNumberOfRequiredParameters();
        }
        $r = new ReflectionFunction($callable);
        return $r->getNumberOfRequiredParameters();
    }

 /**
  * Dispatch middleware
  *
  * Given a route (which contains the handler for given middleware),
  * the $err value passed to $next, $next, and the request and response
  * objects, dispatch a middleware handler.
  *
  * If $err is non-falsy, and the current handler has an arity of 4,
  * it will be dispatched.
  *
  * If $err is falsy, and the current handler has an arity of < 4,
  * it will be dispatched.
  *
  * In all other cases, the handler will be ignored, and $next will be
  * invoked with the current $err value.
  *
  * If an exception is raised when executing the handler, the exception
  * will be assigned as the value of $err, and $next will be invoked
  * with it.
  *
  * @param Route $route
  * @param mixed $err
  * @param ServerRequestInterface $request
  * @param ResponseInterface $response
  * @param callable $next
  */
 public function __invoke(Route $route, $err, ServerRequestInterface $request, ResponseInterface $response, callable $next)
 {
     $handler = $route->handler;
     $hasError = null !== $err;
     switch (true) {
         case $handler instanceof ErrorMiddlewareInterface:
             $arity = 4;
             break;
         case $handler instanceof MiddlewareInterface:
             $arity = 3;
             break;
         default:
             $arity = Utils::getArity($handler);
             break;
     }
     // @todo Trigger event with Route, original URL from request?
     try {
         if ($hasError && $arity === 4) {
             return $handler($err, $request, $response, $next);
         }
         if (!$hasError && $arity < 4) {
             return $handler($request, $response, $next);
         }
     } catch (Throwable $throwable) {
         return $next($request, $response, $throwable);
     } catch (Exception $exception) {
         return $next($request, $response, $exception);
     }
     return $next($request, $response, $err);
 }