compiling php as an module

Hi out there, there is a shared module in my distribution in /usr/lib/apache-extramodules/ called libphp4.so. (It is a Mandrake 8.0, using php 4.0.6). The PdfLib is not compiled into this module. So I build the pdflib to /usr/local/ and tried to recompile php using --with-pdflib=/usr/local. It works - but I cannot compile it to the above shared module. How can I realize this, so that I simply can replace it by the new one. Any hints? Thanx. Joern