今天着实是出了点岔子。这是一道课堂练习题目，目标仅仅是提交表单+验证表单的某些地方是否和规矩。
突然脑子想岔了，想实现的是……
将表单提交，并且验证表单的可行性，将表单提交到数据库，返回身份令牌/身份凭证，在网页的每一页都调用它。

为什么这么想？因为有个人突然提了一嘴：期末要交一整个动态网站……

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>表单</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        body {
            background-image: linear-gradient(to right, #fa709a 0%, #fee140 100%);
        }

        #top {
            height: 10px;
            width: 100%;
        }

        h1 {
            margin-bottom: 10px;
        }

        form {
            width: 300px;
            height: 290px;
            background-color: red;
            color: rgb(255, 255, 255);
            margin: 100px auto;
            text-align: center;
        }

        table {
            width: 100%;
            margin-left: 10px;
        }

        table td,
        table {
            border: 1px solid transparent;
        }

        table tr td:nth-of-type(2n-1) {
            text-align: right;
            padding-right: 10px;
        }

        table tr td:nth-of-type(2n) {
            text-align: left;
        }

        #submit {
            margin-top: 10px;
            width: 100px;
            height: 2em;
        }
    </style>
</head>

<body>
    <form action="sqldata.php" method="post">
        <div id="top"> </div>
        <h1>用户注册页面</h1>
        <table>
            <tr>
                <td>用户名:</td>
                <td><input type="text" name="username"></td>
            </tr>
            <tr>
                <td>密码:</td>
                <td><input type="password" name="password" placeholder="字母数字下划线"></td>
            </tr>
            <tr>
                <td>用户类型:</td>
                <td>
                    <select id="gender" name="user_type" required>
                        <option value="0">学生</option>
                        <option value="1">教师</option>
                        <option value="2">管理员</option>
                    </select>
                </td>
            </tr>
            <tr>
                <td>邮箱:</td>
                <td><input type="email" name="email"></td>
            </tr>
            <tr>
                <td>身份证:</td>
                <td><input type="number" name="user_id" placeholder="必须是18位数字"></td>
            </tr>
            <tr>
                <td>手机号码:</td>
                <td><input type="number" name="mobile_number"></td>
            </tr>
            <tr>
                <td>用户地址:</td>
                <td><input type="text" name="user_addr"></td>
            </tr>

        </table>
        <input type="submit" value="注册" name="submit" id="submit">
        <?php
        if (isset($_POST["submit"])) {
            $verify = 0;
            $str = '';
            $username = $_POST['username'];
            $password = $_POST['password'];
            $user_type = $_POST['user_type'];
            $email = $_POST['email'];
            $user_id = $_POST['user_id'];
            $mobile_number = $_POST['mobile_number'];
            $user_addr = $_POST['user_addr'];

            if (($username == "") | strlen(strlen($username) < 3) | $username == null) {
                $str .= '用户名不能空且长度必须大于3！';
                $verify = -1;
            }
            if (preg_match_all('/[0-9]*/', $password  < 1) & preg_match_all('/[a-zA-Z]*/', $password) < 1 | strlen($password) > 16 | strlen($password) < 3 | preg_grep('/[^0-9a-zA-Z]/', $password) != 0) {
                $str .= "密码必须是字母数字组合且长度必须在8到16之间！";
                $verify = -1;
            }
            if (preg_match_all('/[@\.]/', $email) < 1 | preg_match_all('/[@\.]/', $email) > 3) {
                $str .= "电子邮件格式输入错误!";
                $verify = -1;
            }
            if (strlen($user_id) != 18) {
                $str .= "身份证号码必须等于18位!";
                $verify = -1;
            }
            if (strlen($mobile_number) != 11) {
                $str .= "电话号码必须等于11位！";
                $verify = -1;
            }
            $flag = 0;
            if ($verify == -1) {
                $flag = -1;
                echo "<script>alert(' 注册失败');</script>";
                echo "<script>alert(\"" . $str . "\");</script>";
            } else {
                $flag = 1;
                echo "<script>alert('注册成功');</script>";
            }
            if ($flag == 1) {
                $conn = mysqli_connect("127.0.0.1", "root", "123456", "mydatabase");

                // 这里写入数据库
                $tableName = "users";
                $result = mysqli_query($conn, "SHOW TABLES LIKE '$tableName'");
                mysqli_num_rows($result) == 1;
                $sqltable = "INSERT INTO users (username,password,user_type,email,user_id,mobile_number,user_addr) VALUES ('$username','$password',$user_type,'$email','$user_id','$mobile_number','$user_addr')";
                mysqli_query($conn, $sqltable);
                mysqli_close($conn);
            }
        }
        ?>
    </form>
</body>

</html>