How to use PHP to implement a simple registration and login system (source code attached)

<h2>用户注册登录系统</h2>
<hr>

<?php     include_once("function/database.php");
    // $userName = $_POST[&#39;userName&#39;];
    // $password = $_POST[&#39;password&#39;];
    $userName = addslashes($_POST[&#39;userName&#39;]);
    $password = addslashes($_POST[&#39;password&#39;]);
    getConnect();
    $loginSQL = "select * from users where userName=&#39;$userName&#39; and password=&#39;$password&#39;";
    echo $loginSQL;
    $resultLogin = mysql_query($loginSQL);
    if (mysql_num_rows($resultLogin) > 0) {
        echo "登录成功";
    } else {
        echo "登录失败";
    }
    closeConnect();
?>

<?php     include_once("function/fileSystem.php");
    include_once("function/database.php");

    if (empty($_POST)) {
        exit("您提交的表单数据超过post_max_size! <br>");
    }

    // 判断输入密码与确认密码是否相同
    $password = $_POST['password'];
    $confirmPassword = $_POST['confirmPassword'];
    if ($password != $confirmPassword) {
        exit("输入的密码与确认密码不相等！");
    }

    $userName = $_POST['userName'];
    $domain = $_POST['domain'];
    $userName = $userName . $domain;

    // 判断用户名是否重复
    $userNameSQL = "select * from users where userName = '$userName'";
    getConnect();
    $resultSet = mysql_query($userNameSQL);
    if (mysql_num_rows($resultSet) > 0) {
        exit("用户名已被占用，请更换其他用户名");
    }

    $sex = $_POST['sex'];
    if (empty($_POST['interests'])) {
        $interests = "";
    } else {
        $interests = implode(";", $_POST['interests']);
    }

    $remark = $_POST['remark'];
    $myPictureName = $_FILES['myPicture']['name'];

    $registerSQL = "insert into users values(null, '$userName', '$password', '$sex', '$interests', '$myPictureName', '$remark')";
    $message = upload($_FILES['myPicture'], "uploads");

    if ($message == "上传成功" || $message == "没有上传") {
        mysql_query($registerSQL);
        $userID = mysql_insert_id();
        echo "注册成功<br>";
    } else {
        exit($message);
    }

    $userSQL = "select * from users where user_id = '$userID'";
    $userResult = mysql_query($userSQL);
    if ($user = mysql_fetch_array($userResult)) {
        echo "您的注册用户名为：" . $user['userName'];
    } else {
        exit("用户注册失败！");
    }
    closeConnect();

<?php     $databaseConnection = null;
    function getConnect() {
        $hosthome = "localhost";
        $database = "register";
        $userName = "root";
        $password = "123456";
        global $databaseConnection;
        $databaseConnection = @mysql_connect($hosthome, $userName, $password) or die (mysql_error());
        mysql_query("set names gbk");
        @mysql_select_db($database, $databaseConnection) or die (mysql_error());
    }
    
    function closeConnect() {
        global $databaseConnection;
        if ($databaseConnection) {
            @mysql_close($databaseConnection) or die (mysql_error());
        }
    }
?>

<?php     function upload($file, $filePath) {
        $error = $file[&#39;error&#39;];
        switch ($error) {
            case 0:
                $fileName = $file[&#39;name&#39;];
                $fileTemp = $file[&#39;tmp_name&#39;];
                $destination = $filePath . "/" . $fileName;
                move_uploaded_file($fileTemp, $destination);
                return "上传成功";
            case 1:
                return "上传超过upload_max_filesize";
            case 2:
                return "上传文件超过form的MAX_FILE_SIZE";
            case 3:
                return "附件部分上传";
            case 4:
                return "没有上传";
        }
    }
    ?>