php入门教程之用户认证

< !DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" > 
< html  xmlns ="http://www.w3.org/1999/xhtml" > 
< head > 
< meta  http-equiv ="Content-Type"  content ="text/html; charset=utf-8"  /> 
< title >用户认证--注意没有加密 </title> 
</head> 
< body > 
<?php 
session_start(); 
mysql_connect("localhost","root","123456"); 
mysql_select_db("test"); 
if(isset($_POST['logout'])) { 
$_SESSION = array(); 
session_destroy(); 
echo "您已经成功退出! 单击  < a  href ='user.php' >这里 </a> 继续!"; 
}else{ 
  if(isset($_SESSION['logged'])) { //如果用户已经登录 
  echo "欢迎 ".$_SESSION['username']; //显示欢迎信息 
?> 
< form  method ='POST'  action ='user.php' > 
< input  type ='submit'  value ='注销'  name ='logout' > 
</form> 
<?php     
  }else{ //否则没有登录 
    if(isset($_POST['submit'])&&isset($_POST['usr'])&&isset($_POST['pwd'])) { 
             if(mysql_num_rows(mysql_query("SELECT id FROM accounts WHERE username = '".$_POST['usr']."' && password = '".$_POST['pwd']."' ")) > 0 ) { 
                        $_SESSION['logged'] = true; 
                        $_SESSION['username'] = $_POST['usr']; 
                        $_SESSION['password'] = $_POST['pwd']; 
                        echo " < font  color ='green' >你已经成功登录! </font>"; 
                        header("refresh:3;url='user.php'");//3秒后跳转 
            }else{    
                        echo " < font  color ='red' >登录失败.请重试! </font>"; 
                        header("refresh:1;url='user.php'"); 
            } 
    }else{ //如果用户没有按下submit提交按钮，则显示此表单 
?>    
< form  method ='POST'  action ='' > 
< b >用户名: </b>  < input  type ='text'  name ='usr' > < br > 
< b >密码: </b>  < input  type ='password'  name ='pwd' > < br > 
< input  type ='submit'  value ='login'  name ='submit' > 
</form>    
<?php 
    } 
  }//用户未登录 
} 
?> 
</body> 
</html>