如何利用PHP后端功能开发实现数据库操作？

$servername = "localhost"; //数据库服务器地址
$username = "root"; //数据库用户名
$password = "123456"; //数据库密码
$dbname = "test"; //数据库名称

//创建连接
$conn = mysqli_connect($servername, $username, $password, $dbname);

//检测连接是否成功
if (!$conn) {
    die("连接失败: " . mysqli_connect_error());
}
echo "连接成功";

//查询语句
$sql = "SELECT id, name, age FROM users";

//执行查询操作
$result = mysqli_query($conn, $sql);

//处理查询结果
if (mysqli_num_rows($result) > 0) {
    while ($row = mysqli_fetch_assoc($result)) {
        echo "ID: " . $row["id"]. " - Name: " . $row["name"]. " - Age: " . $row["age"]. "<br>";
    }
} else {
    echo "没有查询到数据";
}

//释放内存
mysqli_free_result($result);

//插入语句
$sql = "INSERT INTO users (name, age, email) VALUES ('John Doe', 25, 'john@example.com')";

//执行插入操作
if (mysqli_query($conn, $sql)) {
    echo "数据插入成功";
} else {
    echo "数据插入失败: " . mysqli_error($conn);
}

//更新语句
$sql = "UPDATE users SET age = 30 WHERE name = 'John Doe'";

//执行更新操作
if (mysqli_query($conn, $sql)) {
    echo "数据更新成功";
} else {
    echo "数据更新失败: " . mysqli_error($conn);
}

//删除语句
$sql = "DELETE FROM users WHERE age < 18";

//执行删除操作
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);
echo "连接已关闭";