I tried to build the php3 module using the DSO support in apache (version 1.3.3), but I was unable to run the server with the dynamically loaded module. The server reported a unresolved dependency in the php shared object.

Testing the configuration gives the following:
> apachectl configtest 
load /usr/local/apache/libexec/libphp3.so into server: /usr/local/apache/libexec/libphp3.so: undefined symbol: get_field_val

When I run ldd on libphp3 I don't get any unresolved dependencies.
I configured php with --with-shared-apache=... and it worked fine. I eventually got apache to build using the --activate-module=.. and --enable-shared=php3 options, and I could start the server ok *without* the AddModule line for libphp3.so. With the AddModule line for php3 included the server would not restart (see above). I rebuilt apache using a static configuration for php and it appears to work ok.