Sessions:
  When a person visits ANY of your pages,
  you magically have an array (named $_SESSION) which contains
  whatever you'd saved in it, last time THAT PARITICULAR USER
  visited any page on your site.
  (Every different repeat-user has different $_SESSION contents.)

How to invoke such magic?
  Just call `session_start` to start (or, resume) a session.
  That's all!
  
How does it work?
  There is one cookie -- think "customerID" -- from the user;
  when you call `session_start` it looks at the customerID and
  finds an associated file-on-disk, and loads that file's info 
  into the array $_SESSION.
  
  If you write into that array, php (when you finish the program)
  will automatically update that-session's-file with the new info.

Practical Issue:
  SESSION array can work great, esp. for things like a shopping-cart's contents.
  
  BUT they're not always the best way to remember info -- esp. if multiple tabs
    could confuse the issue.
  Consider:
  (Think airline searches, and showing a single 'purchase' link on the result.)
  If the info depends on which "tab" the user is in -- then beware
  on SESSION contains the most-recent-info, but user might be wanting
  info from a differnt tag and want to use the second-most-recent-info.
  In that case:
  A solution for this could be 'hidden' input tags:

  Your flight#1197 would leave on Tues and cost $45.
  <form>
     <input type='hidden' name='flight-number' value='1197'/>
     <input type="submit" value="do you want to buy it?"/>
  </form>