Today when I trying to add a new function on my WordPress website, the code I create have no any display, but there is a new warning message pop up:
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'change page_content' not found or invalid function name in
The warning meaning is very clearly, the system call the function I defined, but the it can not found the function named “change page_content“.
Solution
Of course, a normal function name can not have any space. So I rename a new function named change_page_content.
After I fixing it, my program worked.
So, if you encountered this problem, maybe you can check your function name.