If you're talking about your answer (now deleted) to in drupal's theme_form_element function, array += operator throws Unsupported operand error, then it was immediately downvoted for two reasons:
Firstly, the initial line itself is wrong, and it makes the rest of the post meaningless in this case. If you don't know about the operator that the question was talking about, we can reasonably assume that you can't answer questions about it.
+= with an array does not concatenate, that is only for adding numbers like so:
Also, it's rather short. It takes very little time to skim over it and realize that it doesn't have much content.