如何避免PHP文件中的SQL注入攻击？

//连接数据库
$pdo = new PDO('mysql:host=localhost;dbname=test', 'username', 'passWord');
//预处理SQL语句
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username AND password = :password');
//绑定参数
$stmt->bindParam(':username', $username);
$stmt->bindParam(':password', $password);
//执行查询
$stmt->execute();

//连接数据库
$conn = mysqli_connect('localhost', 'username', 'password', 'test');
//参数化查询
$stmt = mysqli_prepare($conn, 'SELECT * FROM users WHERE username = ? AND password = ?');
mysqli_stmt_bind_param($stmt, 'ss', $username, $password);
mysqli_stmt_execute($stmt);

//使用htmlspecialchars()过滤用户输入
$username = htmlspecialchars($_POST['username'], ENT_QUOTES, 'UTF-8');
$password = htmlspecialchars($_POST['password'], ENT_QUOTES, 'UTF-8');
//查询用户
$stmt = $pdo->query("SELECT * FROM users WHERE username = '{$username}' AND password = '{$password}'");

$username = $_POST['username'];
if (is_string($username)) {
    //将字符串用引号括起来
    $username = "'" . $username . "'";
}
$password = $_POST['password'];
if (is_string($password)) {
    //将字符串用引号括起来
    $password = "'" . $password . "'";
}
//查询用户
$stmt = $pdo->query("SELECT * FROM users WHERE username = {$username} AND password = {$password}");