As Johno said, putting session_start(); above the include "config.php"; should work.
Code:<?php
session_start();
include "config.php";
?>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<link href="style.css" rel="stylesheet" type="text/css" />
<html>
<head>
<title>The site</title>
</head>
<body>
<center>
<p>R&T CARTS </p>
<br>
<p>
<?php
//REST OF PHP HERE
?>
