mysqli显示错误

$conn = mysqli_connect("localhost", "username", "password", "database");
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

$conn = mysqli_connect("localhost", "username", "password", "database");
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

$result = $conn->query("SELECT * FROM table_name");
if ($result) {
    while ($row = $result->fetch_assoc()) {
        // 处理结果集
    }
    $result->free();
}
$conn->close();

$conn = mysqli_connect("localhost", "username", "password", "database");
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

$stmt = $conn->prepare("SELECT * FROM table_name WHERE id = ?");
$stmt->bind_param("i", $id);

$id = 1;
$stmt->execute();

$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    // 处理结果集
}

$stmt->close();
$conn->close();