cURL: Uncaught Error: Object of class stdClass could not be converted to string

Not sure why I'm getting the following error:

Uncaught Error: Object of class stdClass could not be converted to string

I am following the tutorial here:

https://www.youtube.com/watch?v=ZIsdbVOQJNc&t=20s

Around the 6:16 minute mark is where I'm at.

Here is my code:

<?php
  $url = "http://localhost/restapi/";

  $data_array = array(
    'username' => 'jbeasley',
    'fullname' => 'John Beasley',
    'email' => 'jbeasley@email.com'
  );

  $data = http_build_query($data_array);

  $ch = curl_init($url);

  curl_setopt($ch, CURLOPT_URL, $url);
  curl_setopt($ch, CURLOPT_POST, true);
  curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

  $resp = curl_exec($ch);

  if($e = curl_error($ch)) {
    echo $e;
  } else {
    $decoded = json_decode($resp);
    foreach($decoded as $key => $val) {
        echo $key . ': ' . $val . '<br>';  // <-- error is pointing here
    }
  }

  curl_close($ch);
?>

The error is pointing at the $val variable in the foreach loop.

The actual API looks like this:

<?php
  require_once __DIR__ . '/config.php';
  class API {
    function Select() {
        $db = new Connect;
        $users = array();
        $data = $db->prepare('SELECT * FROM users');
        $data->execute();
        while($OutputData = $data->fetch(PDO::FETCH_ASSOC)) {
            // $users[$OutputData['username']] = array(
            $users[] = array(
                'username' => $OutputData['username'],
                'fullname' => $OutputData['fullname'],
                'email' => $OutputData['email']
            );
        }

        return json_encode($users);
    }
  }

  $API = new API;
  header('Content-Type: application/json');
  echo $API->Select();
?>

What am I doing wrong and how can I fix it?

Using the suggestion by aynber, I was able to solve the problem by updating the foreach loop to look like this:

foreach($decoded as $key => $val) {
  foreach($val as $key2 => $val2) {
    echo $key2 . ': ' . $val2 . '<br>';
  }
}