如果字符串中的变音符号/变异，PHP中的输出失败（无输出！）

If i read a text from Database, which contains ü, ö, ä, and so on -> my function gives NO output.

Its not so easy to check errors because its a WebHook called script.

Example (like my function):

 function foo() {

    global $db_link;
    $output = "Names: ";

    $res = mysqli_query($db_link, "SQL");

    while ($data = mysqli_fetch_assoc($res)) {
    $output .= "Desc1: ".$data['name1']."/";
    }

    return $output;
    }

Here is a part of my original function:

    $sql = "SELECT * FROM `member_cars` WHERE `car_memberid` = ".$profile_data['member_id'];
    $res = mysqli_query($db_link_website, $sql);
    $anzahl_erg = mysqli_num_rows($res);

if ($anzahl_erg>0) {
        $return_var .= "❚ ".$anzahl_erg." Fahrzeug(e):
    ";  

    while ($data = mysqli_fetch_assoc($res)) {


        if ($i==2) { echo $return_var .= "
        ".$i." Fahrzeug:
        "; } 

        $return_var .= "
        ".$data['car_displayname']."
        • Hersteller: ".$data['car_hersteller']."
        • Modell:     ".$data['car_modell']."
        • Hubraum:    ".$data['car_hubraum']."
        • Baujahr:    ".$data['car_baujahr']."
        • Leistung:   ".$data['car_ps']."
        • Treibstoff: ".$data['car_treibstoff']."
        ";
                $i ++;
                }

    }