I would like to have a php code which will exclude the oldest post from the certain category of certain taxonomy if total number of posts in this category (in the time of adding a new post) greater than X.

E.g. I have taxonomy ‘post_importance’ and category in it with terms ‘featured’. I already have 3 posts in category ‘featured’ and I am adding the fourth one. I want that after that the oldest post in category would be excluded from it. How to do it (sorry I have no expertise in php whatsoever, just html/css) 🙁

Thanks.