如何使用php将数据库中的属性更新为null值

I'm fairly new to php and mySQL. I'm writing a program where I need to occupy and free spots, the way I did it is I made a table called "spaces" it has the "id" and "occupied_by"(users who occupy this spot) as attributes. I set the deafult value for "occupied_by" to NULL, so this way I will know if the space is occupied or not. I wrote the code that occupies Spots and it works just fine. But I keep getting errors on the code the empties them

in the constrants.php file I have these

define ('SUCCESS', 301);
define ('FAIL', 302);
define ('FREE', 303);

then in the operations.php file I have this code

public function freeSpot($id){

            if(!$this->isOccupied($id)){
                return FREE;
            }
            else {
                $stmt = $this->con->prepare("UPDATE spaces SET occupied_by = NULL WHERE id = ?");
                $stmt->bind_param("ss", $occupiedBy, $id);
                if ($stmt->execute()){
                    return SUCCESS;
                }else{

                    return FAIL;
                }   
            }
}

private function isOccupied($id){

            $query = $this->con->prepare("SELECT occupied_by FROM spaces WHERE (id = ?) LIMIT 1");
            $query->bind_param("s", $occupiedBy);
            $query->bind_result($occupy);
            $query->execute();
            $query->store_result();
            return $query->num_rows > 0;
        }

and in the index.php file which I use to check the code on POSTMAN I have this code:

$app->post('/freeSpot/{id}', function (Request $request, Response $response) {


        $pSpaceId = $request->getAttribute('id');
        $requestData = $request->getParsedBody();
       // $occupiedBy = $requestData['occupied_by'];

        $db = new DbOperations();
        $result = $db->freeSpot($id);

        $responseData = array();

        if ($result == SUCCESS) {
            $responseData['error'] = false;
            $responseData['message'] = 'spot emptied successfully';

            $response->write(json_encode($responseData));
            return $response
                ->withHeader('Content-type', 'application/json')
                ->withStatus(200);

        } else if($result == FAIL) {
            $responseData['error'] = true;
            $responseData['message'] = 'Could not empty';

            $response->write(json_encode($responseData));
            return $response
                ->withHeader('Content-type', 'application/json')
                ->withStatus(200);
        }

        else if($result == FREE){
            $responseData['error'] = true;
            $responseData['message'] = 'spot is already empty';

            $response->write(json_encode($responseData));
            return $response
                ->withHeader('Content-type', 'application/json')
                ->withStatus(200);
        }


        return $response
            ->withHeader('Content-type', 'application/json')
            ->withStatus(422);

});

Whatever id I enter it keeps telling me that the "spot is already empty" while it is not. What's wrong?