使用SlimPHP检索帖子属性

I'm trying to rebuild my PHP API with SlimPHP v3 + PDO. The problem is I'm stuck with retrieving attributes passed in POST method. Here is part of my index.php file, I decided just to echo the variable to test if it's working.

<?php

use \Psr\Http\Message\ServerRequestInterface as Request;
use \Psr\Http\Message\ResponseInterface as Response;

require '../vendor/autoload.php';
require_once '../includes/UserOperation.php';
require_once '../includes/TweetOperation.php';

$app = new \Slim\App([
    'settings' => [
        'displayErrorDetails' => true
    ]
]);

//---UserOperation.php---//

//registering a new user
$app->post('/register', function (Request $request, Response $response) {
    if (isTheseParametersAvailable(array('name', 'email', 'password', 'picture_path'))) {
        $requestData = $request->getParsedBody();
        $name = $request->getParsedBody()['name'];
        $email = $requestData['email'];
        $password = $requestData['password'];
        $picture_path = $requestData['picture_path'];

        echo "Hello " .$name;
}
});

//function to check parameters
function isTheseParametersAvailable($required_fields)
{
    $error = false;
    $error_fields = "";
    $request_params = $_REQUEST;

    foreach ($required_fields as $field) {
        if (!isset($request_params[$field]) || strlen(trim($request_params[$field])) <= 0) {
            $error = true;
            $error_fields .= $field . ', ';
        }
    }

    if ($error) {
        $response = array();
        $response["error"] = true;
        $response["message"] = 'Required field(s) ' . substr($error_fields, 0, -2) . ' is missing or empty';
        echo json_encode($response);
        return false;
    }
    return true;
}

$app->run();

The link for my website looks for example like below,but $name variable that I'm trying to echo remains empty.

https://myweblink/register?name=test&email=test@mail.com&password=pass&picture_path=lolololo