$_SESSION variable help please

well I have my php working somewhere in my localhost, but it didn't pass
value using $_POST or $_SESSION
i also don't know if : text/css is on or working or any of this is working
if(isset($_GET["n"])) $n=$_GET["n"];
if(isset($_GET["letters"])) $letters=$_GET["letters"];
if(!isset($letters)) $letters="";
if(isset($PHP_SELF)) $self=$PHP_SELF;
else $self=$_SERVER["PHP_SELF"];

also I printed out my php info and my where it says :
PHP_SELF it says (no value) so I guess this is on or something.