First, check to see if your PHP Live! version is v.4.7.9.9.8.9.  There was a known issue with that version regarding this type of error.  If your system is v.4.7.9.9.8.9, simply download the latest version and update your system.  Upgrading your system to the latest version will resolve the issue.  The latest version can be downloaded from the client area and from the free version page.

If your system is not v.4.7.9.9.8.9, it is most likely a custom language pack creation issue.  Follow the steps detailed below to correct the issue.

If you receive an error similar to "Use of undefined constant LANG_CHAT_.. - assumed 'LANG_CHAT_.." or "Update language pack", it is due to the custom language file that has been created and the latest build of PHP Live! is not able to retrieve the proper variables from the custom language file.

1. Copy an existing language file from the phplive/lang_packs/ directory and rename the copied language file to a new name and proceed to edit the new file for your needs.
2. Update your language settings at the Setup Admin > Departments to use the new language file you just created.
3. Done!