Consistently iterate API response which may be an object or an array of objects

You have the following json

{
  "Artikelnr":"1000",
  "Vooraad_NL":2.0,
  "Voorraad_DE":1.0,
  "Voorraad_BE":0.0
}

So, when you looping it after json_decode, you are getting something like this:

Array
(
    [Artikelnr] => 1000
    [Vooraad_NL] => 2
    [Voorraad_DE] => 1
    [Voorraad_BE] => 0
)

So, when you are looping the $data:

$data = json_decode(file_get_contents('php://input'), true);

foreach ($data as $key => $val) {
    $articlenumber = $data['Artikelnr'];
    $voorraad_nl = $data['Vooraad_NL'];
    $voorraad_de = $data['Vooraad_DE'];
    $voorraad_be = $data['Vooraad_BE'];
}

The loop is iterating over the $data array which contains 4 items and you are not using the $key/$val inside your loop but the $data itself so your seeing the same items 4 times.

if you have only one json then you don't need to loop, you can simply do the same thing without the loop, for example:

echo $data['Artikelnr'];
echo $data['Vooraad_NL'];
echo $data['Voorraad_DE'];
echo $data['Voorraad_BE'];

If you are not sure whether the result is an array of objects or a single object, you may try something like this:

// Decode without passing the true in the json_encode
$data = json_decode(file_get_contents('php://input'));

if (is_object($data)) {
    $data->Artikelnr;
    $data->Vooraad_NL;
    $data->Vooraad_DE;
    $data->Vooraad_BE;
} else {
    foreach ($data as $item) {
        $articlenumber = $item->Artikelnr;
        $voorraad_nl   = $item->Vooraad_NL;
        $voorraad_de   = $item->Vooraad_DE;
        $voorraad_be   = $item->Vooraad_BE;
    }
}

if you want to use each item as an array then you may convert the object (stdClass) using (array), for example:

foreach ($data as $item) {
    $item = (array) $item;
    // Now you can use the item as an array
    $articlenumber = $item['Artikelnr'];
    // ...
}

Same could be possible for the single object, for example:

if (is_object($data)) {
    $data = (array) $data;
    $articlenumber = $data['Artikelnr'];
}

I recommend conditionally converting any encountered objects to a objectArrays and unconditionally writing a loop to iterate the objectArray.

All applications should endeavor to stabilize their input/output data types for more streamlined scripting (a shame that the endpoint that you are hitting doesn't make this consideration).

Code: (Demo)

$data = json_decode($response);
if (is_object($data)) {
    $data = [$data];
}
foreach ($data as $obj) {
    var_dump($obj);
}