in phpmyadmin (or mysql client) and see if you get the correct resultset for options in your select. If the query returns correct result then check the html code (in Firefox right click -> View page source) and check whether there are no html errors also.
You might want to check whether isSelected($item['po'],$batch) function returns correct value.
// combo box
<select name="batch" onchange="submit();">
<option value="-1" >-- Select Adress --</option>
Resource id #4<br />
<b>Notice</b>: Undefined index: po in <b>C:\xampp\htdocs\Innovation\script_shop\stock.php</b> on line <b>48</b><br />
<b>Fatal error</b>: Call to undefined function isSelected() in <b>C:\xampp\htdocs\Innovation\script_shop\stock.php</b> on line <b>48</b><br />
This is mysterious. The code in the snippet you posted seem to be OK (once corrected with suggestions from other posts). Can you post the whole code? I would have to test it in my environment to try to find something.