Is line 11 where you define $dompdf (hard to tell with a copy/paste)?
It seems that the DOMPDF autoload function isn't loading the necessary
class files. It is either having trouble finding the files or is not
being defined. The latter would only happen if an autoload has already
been defined by another script. The former could happen if you didn't
maintain the DOMPDF structure or if the web server doesn't have rights
to the files in /dompdf/include.
-b