I did a yum update of AsteriskNOW a couple of days ago and found that after the upgrade phpmyadmin that I had successfully installed on the system some time ago was giving an error when it tried to start.

After some research I found … ut-errors/. As described in this article it seems during the update the php user permissions were restored to root:apache. I did a
chown -R root:asterisk /var/lib/php/sessions and all is now well.

My knowledge of the nuts and bolts is a little limited so I would like to be sure I have done the right thing. Can anyone comment?

Sounds totally reasonable. So a problem of the underlying OS and not specifically of Asterisk, yes?

My feeling is that when php was upgraded during the update it most likely made the change to the user permissions, does this sound feasible?

Yup, sounds like it could have happened.