You haven't solved the problem on the left hand side of the expressions and the right is needlessly wrapped in double quotes. The right hand sideisn't causing a problem without the single quotes because of a legacy way of displaying arrays within strings, nower days you should use the following syntax for planting array elements within string.
$string = "some stuff {$_POST['item_name']}";
However, for your case this is not necesarry as there is nothing either side of the variable. Recode as:
$paypal['item_name']=$_POST['item_name'];
$paypal['item_number']=$_POST['item_number'];
PS..
Grrr, when's someone going to write a js script to enable block editing in textareas?