title: keyword tags structure
body: <p>Stackoverflow allows a poster to make tags for their questions.</p>
  
  <p>Now say I have some articles and want to give some keyword tags to each.  I'm using PHP and mysql for this. </p>
  
  <p>What is the best database structure to use when doing this?</p>
  
  <p>Will a limit of 5 keywords per article and unlimited keywords cases require different database structures?</p>