Using Switch Statement to Change Image According to Last Digit of Topic ID

I am trying to build a function, which gives out different image-urls, according to the last digit of bbp_get_topic_id().
This is, what i’ve got so far:

function bg_image_topic_titlearea() {
  $letztezifferimgs = substr("'.bbp_get_topic_id().'", -1);
  switch ($letztezifferimgs) {
    case "0":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/10/Blockaden-lösen-2.jpg";
        break;
    case "1":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/09/Das-Leben-ist-ein-Spiel-2.jpg";
        break;
    case "2":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2017/02/Bedürfnisse-erkennen-3.jpg";
        break;
    case "3":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/08/Selbsterkenntnis-sich-selbst-finden2-1.jpg";
        break;
    case "4":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/11/Reinkarnation-Wiedergeburt-2.jpg";
        break;
    case "5":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/10/Intuition3.jpg";
        break;
    case "6":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/07/Manchmal-muss-man-loslassen-3.jpg";
        break;
    case "7":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2016/07/Manchmal-muss-man-loslassen-3.jpg";
        break;
    case "8":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2017/02/Pfad.jpg";
        break;
    case "9":
        echo "https://test.bewusstewelt.de/wp-content/uploads/2017/01/Zirbeldrüse-aktivieren-2.jpg";
        break;
}
}  

Yet it does not react, if i use .bg_image_topic_titlearea(). to get the image url.
I am trying to get it done for some time now and i would be glad, if someone could help me.

Kind regards
Dominik