1. “Failed opening required...” or “Undefined function...”

This most likely indicates that a required file could not be loaded. First check if the file is there and in the correct location. If it is, then check the permissions of the file to make sure the webserver can READ it. See What is CHMOD in the FAQ section.

It is also possible that the file is corrupted, or that your FTP program changed file/directory names. See How do I handle the files? in the FAQ section.

Check your FTP program settings (make sure you “preserve directory structure”, and do NOT “force lowercase”, etc...) and try again installing/uploading the files again.