INSERT INTO，(可能是SQL注入) UPDATE PHP (主键的重复条目'2‘) PHP SQL语句使用表单

// 使用PDO预处理语句
try {
    $pdo = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
    $stmt = $pdo->prepare("INSERT INTO users (username, email) VALUES (:username, :email)");
    $stmt->bindParam(':username', $username);
    $stmt->bindParam(':email', $email);
    $username = $_POST['username'];
    $email = $_POST['email'];
    $stmt->execute();
} catch (PDOException $e) {
    echo "Error: " . $e->getMessage();
}

// 检查主键是否存在
$stmt = $pdo->prepare("SELECT COUNT(*) FROM users WHERE id = :id");
$stmt->bindParam(':id', $id);
$stmt->execute();
$count = $stmt->fetchColumn();
if ($count == 0) {
    // 主键不存在，可以插入
    $stmt = $pdo->prepare("INSERT INTO users (id, username, email) VALUES (:id, :username, :email)");
    $stmt->bindParam(':id', $id);
    $stmt->bindParam(':username', $username);
    $stmt->bindParam(':email', $email);
    $stmt->execute();
} else {
    // 主键存在，处理重复条目
    echo "主键已存在";
}