Problems with mysqli connect? Internal server error 500

Question

I've tried several different connection solutions throughout the day, but I keep experiencing internal server error 500 in one way or the other.

I have made a seperate connection.php document which is the code shown below:

$user = 'root';
$password = 'root';
$db = 'Vilduhelst';
$host = 'localhost';

$mysqli = new mysqli($host, $user, $password, $db);
if ($mysqli->connect_errno) {
    echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error;
}

However, if I try to add the following code to the document, I get an error 500:

function performQuery($sql){
    global $mysqli;
    $result = mysqli_query($mysqli, $sql);

    if ($result){
        return $result;
    } else {
        echo 'Something went wrong'
        return null;
    }
}

Does anybody know why?