session_start(); // старт сессий
$_SESSION['название переменной'] = "значение";
// Например надо вывести имя.
$_SESSION['name'] = 'Test';
echo $_SESSION['name'];

$idUser = $_GET['id'];
$all_items = mysql_query("SELECT * FROM itemPlayer WHERE id=$idUser") or die("Error sql: ".mysql_error());

while($item = mysql_fetch_array($all_items))
{
         echo $item['item_id'];
}

$userID = $_GET['id'];
$sql = mysql_query("SELECT * FROM items WHERE idPitem = $userID ") or die("Error sql: ".mysql_error());

$item = mysql_fetch_array($sql);

$inven_item = $item['item_id'];
$insertItem = mysql_query("INSERT INTO invent ('items_ids') VALUES ('$inven_item') ") or die("Error sql: ".mysql_error());

<?
$mysqlhost = "localhost";
$mysqluser = "root";
$mysqlpass = "";
$mysqlbase = ""; // в скобочках название базы данных
$res = mysql_connect ($mysqlhost,$mysqluser,$mysqlpass,$mysqlbase);
mysql_select_db($mysqlbase, $res); mysql_query("/*!40101 SET NAMES 'cp1251' */") or die("Error: " . mysql_error());

$data = @mysql_connect("localhost","WhiteWolf","passwort");
if(!$data){die('<center><font color=red>Password Incorrect</font></center>');}
@mysql_select_db("База",$data);
if (mysql_error()){die('<center><font color=red>Error select data base</font></center>');}
mysql_query("SET NAMES cp1251");

?>