Another annoying problem we're grappling with while upgrading PHP for a Drupal 7 site is session handling. Authentication is breaking on some browsers across platforms because Drupal's session creation method is silently failing to generate a session.

https://www.drupal.org/node/2385567 <-- this link claims that PHP 5.6.5 fixes this bug, but we're experiencing it on PHP 5.6.7 :(

This is a note about it on php.net https://bugs.php.net/bug.php?id=68331