Syntax error unexpected 'string'

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PHP

MVC

syntax

This error keeps popping up in one of the classes I use. The error is not tied to any other part of the code it just keeps activating on the first line of code in the Application class.

"

public static string $ROOT_DIR;

" is the problem right now. But if I add a line of code above it the error jumps to that line.

class Application{

    public static string $ROOT_DIR;
    public Router $router;
    public Request $request;
    public Response $response;


    public function __construct($rootPath)
    {
        self::$ROOT_DIR = $rootPath;
        $this->router = new Router();
        $this->request = new Request($this->router);
        $this->response = new Response();
    }

    public function run(){
        echo $this->router->resolve();
    }
}

Also, I think, the error has an effect on this part of my MVC.

class Router{

    public Request $request; (syntax error, unexpected request))
    protected array $routes = [];

    public function __construct(\app\core\Request $request)
    {
        $this->request = $request;
    }

What I have tried:

My guess was it's either a typo or a problem generated because I copied some parts of the code around. But typing the liens of code fresh did nothing.