[Quale]

PHP Code:

$_SESSION['username'] = $user; 


don't forget

PHP Code:

session_start(); 


at the start and

PHP Code:

include("sql.php"); 


if you're working with a MySQL database.

SQL.PHP:

PHP Code:

<?php
$connection = mysql_connect('localhost', 'DATABASE_USERNAME', 'DATABASE_PASSWORD');
$database = mysql_select_db('DATABASE_NAME', $connection);
?>

edit:
On logout:

PHP Code:

unset($_SESSION['username']);