I re-started my installation of PHPDOCX from the beginning and the execution of OpenOffice in headless mode was successful.
I had another problem. When I executed my PHP script in a command shell I had the following error :
error while loading shared libraries: libgthread-2.0.so.0: cannot open shared object file: No such file or directory
I solved this problem by installing the package ia32-libs-gtk
And now it works.
Thank you for your help.
Regards