Hi,
AFAIK php support for apache 2 is still some kind of beta. It might work without any problems but as long as you don't need some of the new features of apache 2 I'd suggest to stay with apache 1.3.27.
About the zlib problem:
Depending on where you installed the zlib you might need to add the path to the zlib to the following environment variable if you want to link other software against it:
LD_LIBRARY_PATH.
Another thing to do is to execute ldconfig after adding the path to the zlib library to the /etc/ld.so.conf file (for SuSE Linux, I don't know if it is the same with RH8).