如何正确处理PHP表单提交

<form action="submit.php" method="post">
   <input type="text" name="username" placeholder="请输入用户名" required>
   <input type="password" name="password" placeholder="请输入密码" required>
   <input type="submit" value="提交">
</form>

$username = $_POST['username'];
$password = $_POST['password'];

if(strlen($username) < 6 || strlen($username) > 16){
   echo "用户名长度必须在6-16个字符之间";
   exit;
}
$username = htmlspecialchars($username);

// 连接数据库
$conn = mysqli_connect("localhost", "root", "password", "database");
// 检查连接是否成功
if (!$conn) {
   die("连接数据库失败: " . mysqli_connect_error());
}
// 执行插入操作
$sql = "INSERT INTO users (username, password) VALUES ('$username', '$password')";
if (mysqli_query($conn, $sql)) {
   echo "数据提交成功";
} else {
   echo "数据提交失败: " . mysqli_error($conn);
}
// 关闭数据库连接
mysqli_close($conn);

if(empty($username) || empty($password)){
   echo "用户名和密码不能为空";
   exit;
}

if(strlen($username) < 6 || strlen($username) > 16){
   echo "用户名长度必须在6-16个字符之间";
   exit;
}

$sql = "SELECT * FROM users WHERE username = ? AND password = ?";
$stmt = mysqli_prepare($conn, $sql);
mysqli_stmt_bind_param($stmt, "ss", $username, $password);
mysqli_stmt_execute($stmt);
$result = mysqli_stmt_get_result($stmt);
while ($row = mysqli_fetch_assoc($result)) {
   // 处理结果集
}
mysqli_stmt_close($stmt);

$username = htmlspecialchars($_POST['username']);