Check/note the permissions of:
/var/lib/php/session
In case you need to set them back.
Then perhaps this command will do the job:
chmod 1733 /var/lib/php/session
as per : http://kb.odin.com/en/115704
Although this KB article is specifically for a slightly different issue, is similar potential cause/workaround.
i.e. the script tries to create the session but cannot write it, so login is successful, then the required session file is not there.
If it does fix the problem for you, likely need to reapply this whenever you run yum update and it includes any php update.
