asterisk ippbx not working after yum update

Dear Experts:

We have deployed asterisk ip-pbx for our sales operation with xorcom appliance other than default shipping this required custom coded from the developer  done in PHP. eveything was working fine but recently after yum update the service affected. now that devloper still working to fix.

Developer said update was required for asterisk hence did yum update and this broke the code.

I think instead of  using yum update could have used stand alone appliacation wise update.

Please tell me how to handle such sitation

1.Is it possible to undo the yum update process to restore to previous.
2. what measures the developer should take before going for yum update.
3. how to avoid yum update affecting astersik, php , freepbx and dhadi

Please suggest.