The following prints the expected output on one box:

 putenv("TZ=PST");
 print date("l, jS of F, Y @ H:i (T)");

The other box always presents the system-default timezone. Worse, if I hammer the first box the right way (it seems to be mostly flat-out reloading, but sometimes it just happens immediately), it'll spontaniously display the system-default timezone as well, if only for one page reload.

Both boxes have the timezone data (/usr/lshare/zoneinfo/*), have the same revisions of glibc2 from Debian (2.1.3-13), the same php4/apache packages from Debian, same kernel version (although with different compliation options), etc.. 

Thanks in advance :)