<?php
$my_array=array(1,2,3,4);
// put the array in a session variable
$_SESSION['cart']=$my_array;
// create IN placeholders
$in = str_repeat('?,', count($_SESSION['cart']) - 1) . '?';
$sql1 = "SELECT * FROM product WHERE id IN ($in)";
$params = $_SESSION['cart'] ;
//$stmt = $dbh->prepare($sql1); commented out for demo
//$stmt->execute($params); commented out for demo
echo $sql1, PHP_EOL, print_r($params, true);