e-commerce/add to cart

Question

<?php
$id=$_REQUEST['proid'];  //this id is that product selected

$_SESSION["cart"]=$id;      //get the value in $_SESSION["cart"]
 
//echo $_SESSION["cart"]  //print the value that in session for check value coming in session or not
 
?>
problme is that when i select next product first vale from session overlaped by current proid value.

i want to get the selected proid for cart.php to fetch the product detail from database and want to complet the shopping cart code.

if Respected people have add to cart demo or any link about it plsease help me.