output started at /home/firthweb/public_html/add_inv_create_items.php:1
This means that there is a blank line somewhere or something else being written to the page. A character, even a blank space is usually the "output" it is referring to.
Are there any blank lines in the connection include? If you move that include below, does it still throw an error? You may be able to simply rearrange them? ! I'm really not sure since it doesn't appear you have the common problem that causes this particular issue.