Customizing welcome screen

I need to know how to post a username on my page from my database, but only if the user log's in correctly...can anyone help?

Should be pretty simple :S. Just..


<?
//check if user is logged in
if($loggedin)
{
echo "Welcome".$username."!";
}
else
{
echo "Welcome Guest!";
}
?>


Where $loggedin would be if the session is equal to the value needed to view it.