You're going to have to load and send the fields in a format ExtJS understands. The simplest way, if you don't mind coupling directly to the ExtJS format, would be to generate the ExtJS template style on the backend. In that case, your php script would generate something like this:
xtype: '<?php echo $field ?>',
value: '<?php echo $value ?>'
Obviously, you could rearrange this so the "items" array is built up beforehand in a loop, and you can add as many items as you need.
Also, depending on how you set this up, you can also just return the array of items and add them to the form on the fly, removing the need to send the "new Ext.form.. etc" portion, and decoupling your code a little better. Personally, that's the way I'd go.
In reply to your comment, if you return the JSON array in a proper ExtJS structure, all you'll need to do is use the "panel.add(myItems);"  method, and possibly a call to "panel.doLayout();" to force it to re-render nicely.
Check out the details on "components"  for how the xtype works.
I had the same problem, afaik has to do with selectlist rendering before the items are added to the store. I tried the callback method mentioned above without any luck (guess it would have to be a callback on the selectlist rather than the store).
I added this line after adding items to the store and it works fine:
As noted in the docs, store loading is asynchronous, so you have to do your additional processing within the appropriate callback: