$servername = "xxxx";
$username = "xxxxx";
$password = "xxxxx;
$dbname = "aroslavv";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT SUM(ilosc) AS ilosc FROM wplaty WHERE ID='$ID'";

$conn->close();