Use PHP to implement user login and registration functions

<form action="logincheck.php" method="post">  
    用户名：<input type="text" name="username" />  
    <br />  
    密码：<input type="password" name="password" />  
    <br />  
    <input type="submit" name="submit" value="登陆" />  
          
    <a href="register.php">注册</a>  
</form>

<?php  
    if(isset($_POST["submit"]) && $_POST["submit"] == "登陆")  
    {  
        $user = $_POST["username"];  
        $psw = $_POST["password"];  
        if($user == "" || $psw == "")  
        {  
            echo "<script>alert(&#39;请输入用户名或密码！&#39;); history.go(-1);</script>";  
        }  
        else  
        {  
            mysql_connect("localhost","root","sixx");  
            mysql_select_db("vt");  
            mysql_query("set names &#39;gbk&#39;");  
            $sql = "select username,password from user where username = &#39;$_POST[username]&#39; and password = &#39;$_POST[password]&#39;";  
            $result = mysql_query($sql);  
            $num = mysql_num_rows($result);  
            if($num)  
            {  
                $row = mysql_fetch_array($result);  //将数据以索引方式储存在数组中  
                echo $row[0];  
            }  
            else  
            {  
                echo "<script>alert(&#39;用户名或密码不正确！&#39;);history.go(-1);</script>";  
            }  
        }  
    }  
    else  
    {  
        echo "<script>alert(&#39;提交未成功！&#39;); history.go(-1);</script>";  
    }  
  
?>

<form action="regcheck.php" method="post">  
    用户名：<input type="text" name="username"/>  
    <br/>  
    密　码:<input type="password" name="password"/>  
    <br/>  
    确认密码：<input type="password" name="confirm"/>  
    <br/>  
    <input type="Submit" name="Submit" value="注册"/>  
</form>

<?php  
    if(isset($_POST["Submit"]) && $_POST["Submit"] == "注册")  
    {  
        $user = $_POST["username"];  
        $psw = $_POST["password"];  
        $psw_confirm = $_POST["confirm"];  
        if($user == "" || $psw == "" || $psw_confirm == "")  
        {  
            echo "<script>alert(&#39;请确认信息完整性！&#39;); history.go(-1);</script>";  
        }  
        else  
        {  
            if($psw == $psw_confirm)  
            {  
                mysql_connect("localhost","root","sixx");   //连接数据库  
                mysql_select_db("vt");  //选择数据库  
                mysql_query("set names &#39;gdk&#39;"); //设定字符集  
                $sql = "select username from user where username = &#39;$_POST[username]&#39;"; //SQL语句  
                $result = mysql_query($sql);    //执行SQL语句  
                $num = mysql_num_rows($result); //统计执行结果影响的行数  
                if($num)    //如果已经存在该用户  
                {  
                    echo "<script>alert(&#39;用户名已存在&#39;); history.go(-1);</script>";  
                }  
                else    //不存在当前注册用户名称  
                {  
                    $sql_insert = "insert into user (username,password,phone,address) values(&#39;$_POST[username]&#39;,&#39;$_POST[password]&#39;,&#39;&#39;,&#39;&#39;)";  
                    $res_insert = mysql_query($sql_insert);  
                    //$num_insert = mysql_num_rows($res_insert);  
                    if($res_insert)  
                    {  
                        echo "<script>alert(&#39;注册成功！&#39;); history.go(-1);</script>";  
                    }  
                    else  
                    {  
                        echo "<script>alert(&#39;系统繁忙，请稍候！&#39;); history.go(-1);</script>";  
                    }  
                }  
            }  
            else  
            {  
                echo "<script>alert(&#39;密码不一致！&#39;); history.go(-1);</script>";  
            }  
        }  
    }  
    else  
    {  
        echo "<script>alert(&#39;提交未成功！&#39;); history.go(-1);</script>";  
    }  
?>