我试着用php和mysql创建一个注册/登录系统，但是偶然发现pdo语句有问题。由于我不熟悉pdo语句，所以无法使代码的“fetch”部分正常工作，因此我非常感谢您的帮助。另外，我非常感谢您解释“占位符”变量如何在pdo语句中工作，以及我们如何使用pdo语句中的“fetch”方法检查数据库中是否已经有用户名。
mysql数据库相当简单。它分别包括自动递增的“id”（主键）字段、“firstname”字段、“lastname”字段、“email”字段、“username”字段和“password”字段。
下面是代码的html部分：

<form class="signUp" action="signup.inc.php" method="POST">
<input type="text" name="first" placeholder="First name" class="firstname" required>
<input type="text" name="last" placeholder="Last name" class="lastname" required>
<input type="text" name="email" placeholder="E-mail" class="email" required>
<input type="text" name="username" placeholder="Username" class="username" required>
<input type="password" name="password" placeholder="Password" class="password" required>
<input type="submit" class="submit" name="submit" value="Sign up">
</form>

下面是使用pdo语句进行php验证的糟糕尝试（显然它不起作用）：

<?php

if (isset($_POST['submit'])){
include_once 'init.php';

$first = $_POST['first'];
$last = $_POST['last']
$email = $_POST['email']
$username = $_POST['username']
$password = $_POST['password']

if (empty($first) || empty($last) || empty($email) || empty($username) || empty($password)){
    header("Location: signup.php?signup=empty");
    exit();
} else{
    if (!preg_match("/^[a-zA-Z]*$/", $first) || !preg_match("/^[a-zA-Z]*$/", $last)){
        header("Location: signup.php?signup=invalid");
        exit();
    } else{
        if (!filter_var($email, FILTER_VALIDATE_EMAIL)){
            header("Location: signup.php?signup=invalidEmail");
            exit();
        } else{
            $query = $db->prepare("SELECT * FROM users WHERE username = :username");
            $query->execute(['username' => $username]);
            $results = $query->fetch();

            if ($query->rowCount()>0){
                header("Location: signup.php?signup=usernameTaken");
                exit();
            } else{
                $hashedPassword = password_hash($password, PASSWORD_DEFAULT);
                $query = $db->prepare("INSERT INTO users (firstname, lastname, email, username, password) VALUES ('$first', '$last', '$email', '$username', '$hashedPassword'))";
                $query->execute();
                header("Location: signup.php?signup=success");
                exit();
            }
        }
    }
}

} else{
header("Location: signup.php");
exit();
}

谢谢您！