My host (Dreamhost) is planning an mandatory upgrade to PHP 5.6 soon. I am testing it now to weed out bugs. (Previously running php 5.3)
After switching to php 5.6, I get the following error at the top of our site:
Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
I ran the vqmod installer and also deleted the vqcache, to no effect. Maybe it should be noted that I have a renamed opencart admin folder.
Opencart 1.5.6.4
The domain is http://shop.pacherbs.com/ but I've since reverted to php 5.3 for now.
Thanks for your help!!!

EDIT: Additional Logs:
2016-01-25 11:57:12 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:12 - PHP Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:12 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:12 - PHP Warning: Cannot modify header information - headers already sent by (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/index.php on line 179
2016-01-25 11:57:12 - PHP Warning: Cannot modify header information - headers already sent by (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/system/library/currency.php on line 45
2016-01-25 11:57:14 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:14 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:15 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:15 - PHP Warning: Cannot modify header information - headers already sent by (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/system/library/captcha.php on line 40
2016-01-25 11:57:15 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
2016-01-25 11:57:16 - PHP Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/cmargo5/shop.pacherbs.com/system/database/mysql.php:6) in /home/cmargo5/shop.pacherbs.com/vqmod/vqcache/vq2-system_library_session.php on line 12
EDIT: This was solved in another thread. Here's the solution for those searching:
change in BOTH config.php files:
define('DB_DRIVER', 'mysql');
to:
define('DB_DRIVER', 'mysqli');
and check again !