php mysql no result

I have a problem with some PHP, and MYSQL code.

<?php
if(isset($_GET['article_id']))
{

require('../config.php');

$opdracht = "SELECT * FROM app WHERE article_id = " . $article_id;
$resultaat = mysqli_query($mysqli, $opdracht);

if($rij = mysqli_fetch_array($resultaat))
{
?>

He get the info from a specific ID in the database:

<?php echo $rij['bericht']; ?>

} else {

echo "Fout bij ophalen: <br/>";
echo $opdracht ."<br/>";
echo mysqli_error($mysqli). "<vr/>";

By the echo error i get this:

Warning: mysqli_error() expects parameter 1 to be resource, null given in

Whats go wrong?

Julian