title: How-to build tagging system like stackoverflow
body: <p>I'm implementing a tag system similar to StackOverflow tag system but I just wonder How-to get related tags and define the relationships weights between tags like the list of "Related Tags" in any tag page like this <a href="https://stackoverflow.com/questions/tagged/php">https://stackoverflow.com/questions/tagged/php</a> they define the relationship weight by the co-occurrence between 2 or more tags </p>
  
  <p>How I can do this in PHP/MySQl to define the most related tags for tag "X" and keep all weights up to date as users add more and more posts/questions ?</p>