Make sure that "session.auto_start" is disabled in php.ini.
It should be :
session.auto_start = 0
Don't forget to restart Apache after php.ini will have been changed.
Additionally check that directory /tmp has permissions 1777 and PHP session directory ( that is defined by session.save_path varibale in php.ini) is writable for the Apache user:
# ls -ald /tmp
drwxrwxrwt 25 root root 69632 Apr 21 11:48 /tmp
ls -ald /var/lib/php/session
drwxrwx--- 2 root apache 24576 Apr 7 00:05 /var/lib/php/session