How to find “related items” in PHP

Question

we often see \'related items\'. For instance in blogs we have related posts, in books we have related books, etc. My question is how do we compile those relevency? If it\'s

Answer 1

it can also be based on "people who bought this book also bought"

No matter how, you will need some dort of connection between your items, and they will mostly be made by human beings