Magento: ?___SID=U appearing in some urls

Question

Can someone please explain why ?___SID=U is appearing in some Magento URLs on my site and not others?

I think it has something to do with sessions but I am not entir

Answer 1

I addition to Brendan's answer, the ___SID=U is used in the cache as a placeholder for the session ID. It is replaced by Mage_Core_Model_Url::sessionUrlVar() which in turn calls Mage_Core_Model_Url::sessionVarCallback(). These methods are called from Mage_Core_Block_Abstract::_afterCacheUrl(), which means that any URL found in block output will contain the correct session ID (if needed).

So to get rid of the parameter in your own code the "right way" use this:

$url = Mage::getUrl('some/magento/route'); // might append ___SID parameter
$url = Mage::getModel('core/url')->sessionUrlVar($url); // process ___SID

If the string still displays in the rendered page that is a bug. Are you using some custom caching module, or generating URL's using a non-standard way?