src/Resolver.php
protected function isAbstract($middleware): bool
    {
        return is_string($middleware) &&
            class_exists($middleware) &&
            is_a($middleware, MiddlewareInterface::class, true);
    }
protected function isAbstract($middleware): bool
    {
        return is_string($middleware) &&
            class_exists($middleware) &&
            is_a($middleware, MiddlewareInterface::class, true);
    }
Determines if the given middleware value is a string that can be instantiated.
Note that it does not check an application container.

@param mixed $middleware
@return bool