an ID won't be sent to url

public function editVC(){
    //echo"HELLO";
    if(isset($_GET['userID'])){
        $currentUserID = $_GET['userID'];
    }

    $vaccineID = $_POST['vaccines'];
    $ageID = $_POST['ages'];
    $dcomment=$_POST['drcomment'];

    if(VaccineModel::editDChildVC($vaccineID,$ageID,$dcomment,$currentUserID)) {
        header('Location: https://autovaccines-peldorri.c9users.io/Classes/Views/viewChildVC.php?userID= $currentUserID');
     } else {
         echo $vaccineID;
         echo $dcomment;
         echo $currentUserID;
         echo"cant add";
     }

When it redirects, this is the resulted url

https://autovaccines-peldorri.c9users.io/Classes/Views/viewChildVC.php?userID=%20$currentUserID

I can't seem to figure out why the variable won't be sent to the url, although I have tried echoing the variable $currentUserID and it did echo the correct ID, this means the if condition applies and there is a value in the variable.