php, extract xml attachment from pdf file

I must extract a xml file from a pdf file. The xml file is an attachment of the pdf file. on windows with wamp server i used the pdftk program, but can't use it on server with ubuntu, because it requires the java virtual machine and is not installed and must not be. Is there any library to do this with php? I tried with php functions like file(), fopen(), file_get_contents() but can't get the xml file from it. Thanks for any suggestion!