HTTP错误500 mysql php？

Question

I try to insert some data but i get HTTP ERROR 500 dont know why.I saw the problem starts with when i prepare the query.I try to insert mutiple rows from a txt file.

$conn = new mysqli("localhost", "***", "****", "***");
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}

    $contents = file_get_contents("cars.txt");
    $pollfields = explode(',', $contents);

    $query = "INSERT INTO table (`name`) VALUES (?)";

    $zz = $mysqli->prepare($query);

    $zz ->bind_param("s", $bb);

    $mysqli->query("START TRANSACTION");
    foreach ($pollfields as $bb) {
        $zz->execute();
    }
    $zz->close();
    $mysqli->query("COMMIT");

Answer 1

i need to defined it

$conn = new mysqli("localhost", "***", "****", "***");
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}

    $contents = file_get_contents("cars.txt");
    $pollfields = explode(',', $contents);

    $query = "INSERT INTO table (`name`) VALUES (?)";

    $zz = $conn->prepare($query);

    $zz ->bind_param("s", $bb);

    $conn->query("START TRANSACTION");
    foreach ($pollfields as $bb) {
        $zz->execute();
    }
    $zz->close();
    $mysqli->query("COMMIT");