the key to the error message is:
output started at /home2/harveyhu/public_html/shop/admin/managegallery.php:5
it is saying that the output causing the issue is around line 5 of the managegallery.php poage.
most likely it was caused by adding the error reporting code and leaving a trailing space after the closing php tag:
on line 4.
this is illustrated in the second screen shot of the post I linked to.