add data into a two-dimensional array using input fields

<form action="" method="post">
<input type="hidden" name="action" id="action" value="process" />
Name: <input type="text" name="itemname" id="itemname" /><br />
Value: <input type="text" name="itemvalue" id="itemvalue" /><br />
<input type="submit" value="Add Item" />
</form>
<?php
$kaarten = array();
(
  array("Linksys Cisco EA2700","129,99"),
  array("Apple iPad 4","479,00"),
  array("Linksys Cisco RE1000","54,99")
);

if ($_POST['action']=="process") {
//add item to array now we have to do this AFTER the array has been pre-populated and created
$itemname = $_POST['itemname'];
$itemvalue= $_POST['itemvalue'];
//add it to the actual array
$kaarten[] = array($itemname,$itemvalue);
//note there's a major issue here as you're not passing the array between form posts and so its being reset every post
//need to add in facility to pass items between form posts to allow for it to be an N length array otherwise it'll only ever container a max of 4 elements the 3 pre-defined ones and the one being added
}

foreach($kaarten as $subArray)
{
  echo $subArray[0]; // test
  echo $subArray[1]; // 12.99
}
?>

This is the basics you can look at shunting arrays between form posts your self I recommend either storing it in a session with a create if not exists if at the very top or perhaps json encoding it and posting it between the pages in a hidden field