请用php写一个考试系统

好的，以下是一个简单的考试系统的PHP代码示例：

首先是数据库设计，考试系统需要至少两个表：users和questions。

users表用于存储考生的信息，包括姓名、邮箱、密码等。

questions表用于存储考试题目及答案。

下面是数据库表的结构：

CREATE TABLE `users` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `email` varchar(255) NOT NULL,
  `password` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `questions` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `question` varchar(255) NOT NULL,
  `option1` varchar(255) NOT NULL,
  `option2` varchar(255) NOT NULL,
  `option3` varchar(255) NOT NULL,
  `option4` varchar(255) NOT NULL,
  `answer` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

然后是PHP代码，以下是简单的考试系统的PHP代码示例：

<?php
//连接数据库
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "dbname";
$conn = mysqli_connect($servername, $username, $password, $dbname);

//检查连接是否成功
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

//登录功能
if (isset($_POST['login'])) {
    $email = $_POST['email'];
    $password = $_POST['password'];
    $sql = "SELECT * FROM users WHERE email='$email' AND password='$password'";
    $result = mysqli_query($conn, $sql);
    if (mysqli_num_rows($result) == 1) {
        session_start();
        $_SESSION['email'] = $email;
        header('location: exam.php');
    } else {
        echo "Invalid email or password";
    }
}

//考试功能
if (isset($_POST['submit'])) {
    $score = 0;
    $answers = $_POST['answers'];
    foreach ($answers as $key => $value) {
        $sql = "SELECT * FROM questions WHERE id='$key' AND answer='$value'";
        $result = mysqli_query($conn, $sql);
        if (mysqli_num_rows($result) == 1) {
            $score++;
        }
    }
    echo "Your score is: " . $score;
}

//显示登录表单
if (!isset($_SESSION['email'])) {
    ?>
    <form method="post">
        <label>Email:</label>
        <input type="email" name="email"><br>
        <label>Password:</label>
        <input type="password" name="password"><br>
        <input type="submit" name="login" value="Login">
    </form>
    <?php
}

//显示考试题目
if (isset($_SESSION['email'])) {
    $sql = "SELECT * FROM questions";
    $result = mysqli_query($conn, $sql);
    ?>
    <form method="post">
        <?php
        while ($row = mysqli_fetch_assoc($result)) {
            ?>
            <p><?php echo $row['question']; ?></p>
            <input type="radio" name="answers[<?php echo $row['id']; ?>]" value="1"> <?php echo $row['option1']; ?><br>
            <input type="radio" name="answers[<?php echo $row['id']; ?>]" value="2"> <?php echo $row['option2']; ?><br>
            <input type="radio" name="answers[<?php echo $row['id']; ?>]" value="3"> <?php echo $row['option3']; ?><br>
            <input type="radio" name="answers[<?php echo $row['id']; ?>]" value="4"> <?php echo $row['option4']; ?><br>
            <?php
        }
        ?>
        <input type="submit" name="submit" value="Submit">
    </form>
    <?php
}

//关闭数据库连接
mysqli_close($conn);
?>

此代码包含登录和考试功能，它首先连接到数据库，如果没有登录，则显示登录表单。如果已经登录，则显示考试题目。考生可以选择题目答案并提交，然后系统将计算分数并显示在屏幕上。

请注意，此处的代码仅供参考，实际使用中需要进行适当的修改和完善。