title: What is session key in codeigniter 2 for?
body: <p>I am a newbie of the CI framework. I am trying to understand what session id is for. Here is the description from CI doc:</p>
  
  <p>The user's unique Session ID (this is a statistically random string with very strong entropy, hashed with MD5 for portability, and regenerated (by default) every five minutes)</p>
  
  <p>I am just wondering under what occasions I should use this. </p>
  
  <p>Thanks</p>