PHP 関数を使用してユーザー アカウント管理機能を最適化するにはどうすればよいですか?

<?php
// 连接数据库
$servername = "localhost";
$username = "root";
$password = "123456";
$dbname = "myDB";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 创建用户账户
$username = $_POST['username'];
$password = $_POST['password'];
$email = $_POST['email'];

$sql = "INSERT INTO users (username, password, email) VALUES ('$username', '$password', '$email')";

if ($conn->query($sql) === TRUE) {
    echo "用户账户创建成功";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>

<?php
session_start();

// 连接数据库
$servername = "localhost";
$username = "root";
$password = "123456";
$dbname = "myDB";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 用户登录
$username = $_POST['username'];
$password = $_POST['password'];

$sql = "SELECT * FROM users WHERE username='$username' AND password='$password'";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 登录成功
    $_SESSION['username'] = $username;
    echo "登录成功";
} else {
    // 登录失败
    echo "用户名或密码错误";
}

$conn->close();
?>

<?php
// 连接数据库
$servername = "localhost";
$username = "root";
$password = "123456";
$dbname = "myDB";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 修改密码
$username = $_POST['username'];
$newPassword = $_POST['newPassword'];

$sql = "UPDATE users SET password='$newPassword' WHERE username='$username'";

if ($conn->query($sql) === TRUE) {
    echo "密码修改成功";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>