How PHP implements single sign-on through multi-site shared seeion (code example)

    $back = login($name,$pwd);//执行登陆操作,成功就写入session
    //如果登录成功，进行以下操作流程
    if($back){
            $sessionid = session_id();        
            $key = encode($session,$keyword);//生成安全码
        //输出一个登陆成功提示页，并跳转到请求登陆的站点
    }

//解密
$keydecode($key);
//把当前站点的sessionid设置为传递的sessionid
session_id($_GET['sessionid']);
session_start();

$gb_DBname="test";                        //数据库名称 
$gb_DBuser="root";                        //数据库用户名称 
$gb_DBpass="";                            //数据库密码 
$gb_DBHOSTname="127.0.0.1";               //主机的名称或是IP地址 
$SESS_DBH="";                           //数据库对象
session_module_name("User");            //定义session存储按用户定义的方式
$SESS_LIFE=get_cfg_var("session.gc_maxlifetime");//得到session的最大有效期,也可以自定义
function sess_open($save_path,$session_name){ 
    global $gb_DBHOSTname,$gb_DBname,$gb_DBuser,$gb_DBpass,$SESS_DBH; 
    if(!$SESS_DBH=mysql_pconnect($gb_DBHOSTname,$gb_DBuser,$gb_DBpass)){ 
    echo "MySql Error:".mysql_error().""; 
    die(); 
    } 
    if(!mysql_select_db($gb_DBname,$SESS_DBH)){ 
    echo "MySql Error:".mysql_error().""; 
    die(); 
    } 
    return true; 
} 
function sess_close(){ return true; 
} 
function sess_read($key){ 
    global $SESS_DBH,$SESS_LIFE; 
    $qry="select value from db_session where sesskey = '$key' and expiry > ".time(); 
    $qid=mysql_query($qry,$SESS_DBH); 
    if(list($value)=mysql_fetch_row($qid)){ 
    return $value; 
    } 
    return false; 
}
//写入session信息。保存session信息的数据表名为：db_session
//除了主键自增id，需要的字段如下
//sesskey   sessionid
//values    session值
//expiry    session的到期日期
function sess_write($key,$val){ 
    global $SESS_DBH,$SESS_LIFE; 
    $expiry=time()+$SESS_LIFE; 
    $value=$val; 
    $qry="insert into db_session values('$key',$expiry,'$value')"; 
    $qid=mysql_query($qry,$SESS_DBH); 
    if(!$qid){ 
    $qry="update db_session set expiry=$expiry, value='$value' where sesskey='$key' and expiry >".time(); 
    $qid=mysql_query($qry,$SESS_DBH); 
    } 
    return $qid; 
} 
function sess_destroy($key){ 
    global $SESS_DBH; 
    $qry="delete from db_session where sesskey = '$key'"; 
    $qid=mysql_query($qry,$SESS_DBH); 
    return $qid; 
} 
function sess_gc($maxlifetime){ 
    global $SESS_DBH; 
    $qry="delete from db_session where expiry < ".time(); 
    $qid=mysql_query($qry,$SESS_DBH); 
    return mysql_affected_rows($SESS_DBH); 
} 

session_set_save_handler("sess_open","sess_close","sess_read","sess_write","sess_destroy","sess_gc");