//连接数据库
<?php
$hostname_conn="localhost";
$username_conn="root";
$password_conn="123456";
$database_conn="joke";
$link=mysqli_connect($hostname_conn,$username_conn,$password_conn,$database_conn) or trigger_error(mysql_error(),E_USER_ERROR);
?>

//获取POST数据
<?php
$username=$_POST['username'];
$password=$_POST['password'];
?>

//从数据库中查询用户名与密码
<?php
$sql="select * from  login where username='{$username}' and password = '{$password}'";
$rs=mysqli_query($link,$sql);
$rows=mysqli_fetch_array($rs);
?>

//验证用户名与密码
<?php
if($row) {
     if ($username == $row['username'] && $password == $row['password'])
     {
         echo "登陆成功，正在为你跳转至后台页面";
         header("location:index.html");
     }
 }else{
     echo "账号或密码错误" . "<br/>";
     echo "<a href='login.html'>返回登陆页面</a>";
 }
?>

//验证验证码
<?php
  if(!$_POST['code'])
  {
  echo"验证码不能为空";
  return;
  }
  else if($_POST['code']!=$_SESSION['authcode'])
  {
  echo "验证码错误，请重新输入";
  return;
  }
?>