如何在PHP中使用数据库查询和操作函数？

$servername = "localhost";
$username = "root";
$password = "password";
$dbname = "my_database";

// 创建数据库连接
$conn = mysqli_connect($servername, $username, $password, $dbname);

// 检查连接是否成功
if (!$conn) {
    die("数据库连接失败: " . mysqli_connect_error());
}

// 连接成功，可以执行数据库操作了

$sql = "SELECT * FROM users";
$result = mysqli_query($conn, $sql);

// 遍历结果集
if (mysqli_num_rows($result) > 0) {
    while ($row = mysqli_fetch_assoc($result)) {
        echo "ID: " . $row["id"] . "，用户名: " . $row["username"] . "<br>";
    }
} else {
    echo "没有结果";
}

$sql = "INSERT INTO users (username, password) VALUES ('john', '123456')";
if (mysqli_query($conn, $sql)) {
    echo "数据插入成功";
} else {
    echo "数据插入失败: " . mysqli_error($conn);
}

$sql = "UPDATE users SET password='new_password' WHERE id=1";
if (mysqli_query($conn, $sql)) {
    echo "数据更新成功";
} else {
    echo "数据更新失败: " . mysqli_error($conn);
}

$sql = "DELETE FROM users WHERE id=1";
if (mysqli_query($conn, $sql)) {
    echo "数据删除成功";
} else {
    echo "数据删除失败: " . mysqli_error($conn);
}

// 关闭连接
mysqli_close($conn);