0){
 if(in_array($_GET['product_id'],$_SESSION['compare'])){
  print "Ýòîò òîâàð áûë äîáàâëåí ðàíåå äëÿ ñðàâíåíèÿ!";
 }
 elseif(count($_SESSION['compare'])>1){
  print "Äëÿ ñðàâíåíèÿ óæå äîáàâëåííî äâà òîâàðà!";
 }
 else{
  $_SESSION['compare'][] = $_GET['product_id'];
  print "Òîâàð äîáàâëåí äëÿ ñðàâíåíèÿ";
 }
}
?>