Warning: Undefined variable $link (from mysql to mysqli)

i upgrade my code from mysql to mysqli.

I edit the mysql query line and now comes the error

Warning: Undefined variable $link
Fatal error: Uncaught TypeError: mysqli_query(): Argument #1 ($mysql) must be of type mysqli, null given**

function getSystemparameter($parameter) {
  $ergebnis = mysqli_query($link, "select wert from systemparameter where parameter='$parameter'");
  $zeile = mysqli_fetch_row($ergebnis);
  return $zeile[0];

My Database Connection:

function connectDatabase() {
  include('./serverConfig.php');
  $link = mysqli_connect($SQLURL, $SQLBenutzer, $SQLPasswort) or die ("Keine Verbindung !");
  if (!$link) {
    die('Could not connect: ' . mysqli_error());
  }
  mysqli_select_db($link,$DB) or die ("Kann DB nicht öffnen !");
  return $link;

Nothing idea for a solution.