Okay, the ancient GCC appears to have been the culprit.
It basically took me all night to get gcc 4.4.3 installed on this opensolaris server, but once I did, and recompiled php 5.2.12 + lsapi 5.2, it started serving PHP right away. I'll try the same approach on the production servers.
Maybe this is a good warning for a wiki? GCC 3.4.3 is the compiler issued with opensolaris (amazing, isn't it?), so this might be a problem that someone else runs into as well.