php - Errors with login system.... I cant see a issue -

here code first of all

<?php session_start(); //start session define(admin,$_session['name']); //get user name registered super global variable if(!session_is_registered("admin")){ //if session not registered header("location:login.php"); // redirect login.php page } else //continue current page header( 'content-type: text/html; charset=utf-8' ); ?> 

and i'm getting following errors

notice: use of undefined constant admin - assumed 'admin' in c:\users\andrew\documents\server2server\admin\dashboard.php on line 3

notice: undefined index: name in c:\users\andrew\documents\server2server\admin\dashboard.php on line 3

fatal error: call undefined function session_is_registered() in c:\users\andrew\documents\server2server\admin\dashboard.php on line 4

does know why i'm getting these errors?

here fixed code:

<?php session_start(); //start session define("admin",$_session['name']); //get user name registered super global variable if(!isset($_session['admin'])){ //if session not registered header("location:login.php"); // redirect login.php page } else //continue current page header( 'content-type: text/html; charset=utf-8' ); ?> 

thanks rizier123 :)