流明Spotify API无效客户端

Question

I'm working on an app that allows authentication using Spotify
When trying to get the user's access token, i get a 400 bad request 'invalid_client'.

Here is my JS code which fires whenever the user is redirected to the app. The url is localhost:3000/spotify?code=xxxxxxxxxxx&state=xxxxxxxxx

const myHeaders = new Headers();
      myHeaders.append('Accept', 'application/json');
      myHeaders.append('Access-Control-Allow-Origin', '*');
      myHeaders.append('Content-Type', 'application/json');
      
      //gets the param in the url
      const paramCode = getParameterByName('code', window.location.href);

      const defaultOptions = {
        method: 'POST',
        headers: myHeaders,
        body: JSON.stringify({
          code: paramCode,
        }),
        mode: 'cors',
      };

      // POST request to the backend at http://localhost:8000/api/spotify/oauth
      fetch(LocalAPI.getLocalTokenUrl(), defaultOptions)
        .then((response) => {
          return response.json();
        })
        .then((results) => {
          console.log('token : ', results.access_token);
          // "Client error: `POST https://accounts.spotify.com/api/token` 
          //  resulted in a `400 Bad Request` response:
          // {"error":"invalid_client","error_description":"Invalid client"}"
        });

And here is my backend PHP code which fires whenever it receives a POST request at /api/spotify/oauth
It requests the user's access token but fails. A 400 bad request error is catched and sent to the JS code.

$code = $request->input('code');
        $id = $this->getClientId();
        $secret = $this->getClientSecret();
        
        //encodes clientid and secret in base64
        $token = \base64_encode("$id:$secret");

        $headers = [
            'Authorization' => 'Basic '.$token,
            'Content-Type' => 'application/x-www-form-urlencoded'
        ];

        try {
            $guzzle = new Client();
            $results = $guzzle->post($this->getTokenUrl(), [
                'headers' => $headers,
                'form_params' => [
                    'code' => $code,
                    'redirect_uri' => 'http://localhost:3000/spotify',
                    'grant_type' => 'authorization_code'
                ]
            ]);

        }
        catch (\GuzzleHttp\Exception\ServerException $e) {
            echo json_encode($e->getResponse());
        }
        catch(\GuzzleHttp\Exception\ClientException $e) {
            echo json_encode($e->getMessage());
        }
        catch(\GuzzleHttp\Exception\RequestException $e) {
            echo json_encode($e->getResponse());
        }
        
        echo \GuzzleHttp\json_encode($results);

According to Spotify API documentation :
https://developer.spotify.com/web-api/user-guide/#response-status-codes
My request could be malformed ... I tried messing with the Content-Type and set it to application/json. In this case, no errors are catched but i get a "JSON.parse: unexpected character at line 1 column 1 of the JSON data" in my JS code.

Any suggestion would be appreciated.

Best regards.

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