Keeping a PHP session alive

I get asked this a lot. When you log into a session-based system, and walk away for half an hour, frequently you’ll come back to find it is no longer logged in. How do you keep the session alive when you are not active on the site?

The solution is to regularly “poll” the server with JavaScript and have the server change the session so its expiry is reset.

Each page of the site should have the following JavaScript code. Place it in your shared library if you have one.

window.setInterval(function() {
  var el=document.createElement('img');
  el.src='/sessionRenew.php?rand='+Math.random();
  el.style.opacity=.01;
  el.style.width=1;
  el.style.height=1;
  el.onload=function() {
    document.body.removeChild(el);
  }
  document.body.appendChild(el);
}, 60000);

This creates an image that is 1px*1px and mostly invisible (so it retrieves it from the server immediately). As soon as the data is loaded from the server, it is then deleted. No particular JavaScript library is needed.

Now, on the server, create the following file as sessionRenew.php:

<php
session_start();
$_SESSION['rand']=rand();
echo $_SESSION['rand'];
?>

This opens (or creates) the active session, records a change to it (the ‘rand’ value), and echos that out. The reason you force a change is that some session systems, such as memcache, don’t update the session expiry for simple reads, so you need to make an actual change.

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