It just works! :)

This plugin works for me, whereas the popular alternative plugin “Code Snippets” did not.

I wanted to add php code to functions file to hide specific categories from the sidebar widget.
I sourced the code from here:
Article by Ryadel
“WordPress – How to hide a Category from Front page, Archives, Feeds, Searches and Category Widget”

Many thanks! Now, I feel more comfortable to make php changes whenever I need.