title: Access active sessions in PHP
body: <p>How can I get a list of all active PHP sessions on a server and access them from within one user's instance?</p>
  
  <p>The motivating case is displaying a list of all currently active users on the site, where usernames are stored in each user's PHP session.</p>
  
  <p>Note: I know that I can create my own state via a database (or even the filesystem), but I'm looking for a way to utilize the built-in PHP session mechanisms.</p>