CiviCRM Upgrade

Important lesson today during a CiviCRM upgrade.

Do not save the old copy of CiviCRM code in /sites/all/modules!

If you see an error similar to the following then you almost certainly have two copies of the CiviCRM code -

[08-Apr-2016 18:08:14 Europe/London] PHP Fatal error:  Cannot redeclare class CRM_Core_ClassLoader in /var/www/vhosts/crm-demo.medicaldetectiondogs.org.uk/sites/all/modules/civicrm/CRM/Core/ClassLoader.php on line 36

In my case I had downloaded the new version and saved the old version in /sites/all/modules, e.g.

# cd sites/all/modules
sites/all/modules# wget https://download.civicrm.org/civicrm-4.6.15-drupal.tar.gz
sites/all/modules# mv civicrm civicrm-4.6.13
sites/all/modules# tar xvzf civicrm-4.6.15-drupal.tar.gz

This is very bad. The correct sequence is (substituting the correct CiviCRM version of course)

# cd sites/all/modules
sites/all/modules# wget https://download.civicrm.org/civicrm-4.6.15-drupal.tar.gz
sites/all/modules# mv civicrm /tmp/civicrm-4.6.13
sites/all/modules# tar xvzf civicrm-4.6.15-drupal.tar.gz

Simples. Moving the old code tree outside of the normal code tree solves the problem.