Apache 1.3 supports Dynamic Shared Objects (DSO's) and can unload and load them as necessary, this increases both flexibility and performance. To see if your version supports them, type httpd -l and if mod_so.c is loaded in the list that will appear, than it's ok.
Recompile apache using ./configure -enable-module=most --enable-shared=max. after that you do make and make install and start the apache server using apachectl start
Ok, next step: install php4 and configure it to be a shared apache module; afterwards do ./configure -with-apxs=/usr/local/apache/bin/apxs
In httpd.conf you need to uncomment the next line (or add it if it dones't exist) Addtype application/x-httpd-php .php
and , of course restart the apache server using apachectl restart