PHP：绑定数组值时准备语句的错误消息（MySQLi，SELECT）

I am new to PHP and MySQLi and hope someone can help me with this.

I would like to use a prepared statement for a Select query in order to prevent SQL injection - using MySQLi.

So far I have the below code which returns the following error:
"Call to a member function fetch_assoc() on a non-object...".

However, if I hardcode the $IN params (i.e. a list of integers) and remove the call_user_func_array line then it works as intended.

My PHP:

case "fetchContent":
    $content = $_POST["content"];
    $IN = implode(",", array_fill(0, count($content), "?"));

    $stmt = $conn->prepare("SELECT tID, en FROM TranslationsMain WHERE tID IN($IN) ORDER BY tID");
    call_user_func_array(array($stmt, "bind_param"), $content);
    $stmt->execute();
    $result = $stmt->get_result();
    while($arrTranslations = $result->fetch_assoc()){
        $translations[] = array("tID" => $arrTranslations["tID"], "content" => $arrTranslations["en"]);
    }
    var_dump($translations);
    break;

Note:
$IN creates something like the following - e.g. when $content contains 7 items then dumping $IN returns: string(13) "?,?,?,?,?,?,?"

Update - Example for $content:

array(7) {
  [0]=>
  string(1) "1"
  [1]=>
  string(1) "2"
  [2]=>
  string(1) "3"
  [3]=>
  string(1) "4"
  [4]=>
  string(1) "5"
  [5]=>
  string(1) "6"
  [6]=>
  string(1) "7"
}

Can someone tell me how to resolve this and maybe also provide an explanation why I am getting this error ?

Many thanks for any help,
Mike