I'm not 100% sure of the exact reason, as I have compiled the OCI modules with the Oracle 8.0.5 c modules, and it still didn't work for me.
It's to do with the way that Oracle client accesses the tnsnames.ora file. If you are using PHP4 and Oracle 8.0.5 then you must be running the Oracle server locally because it does not appear to be able to read the tnsnames.ora file in any form what-so-ever. If however you use the Oracle 8.1.6 client you can of course have the Oracle server anywhere you like, as expected, because it is then perfectly capable of reading the tnsnames.ora file.
I may stand corrected, but I believe that this is a Windows platform problem, and I don't think that the same problem exists on Unix/Linux/Solaris platforms.
All I can really suggest is that if you're having problems with Oracle and PHP then change your Oracle client to 8.1.6 and the world will appear round again <g>
Hope this is of some use to some (probably less haired than before) people out there.
TTFN !
Wayne