How to make a recommender system based on tags?

Question

PHP

Each article has tags.
Total articles more than 800k.
the best and fastest way to display relevant articles for each article

Answer 1

can't explain it in a nutshell.
Yes, and such volumes of indices already require certain approaches.
And I could also insert here "it is irrational to do this in PHP", but this is another couple of pages of disputes with especially fanatical PHPists.