Interestingly enough, I tried it on several of my machines, which are different platforms and different (sub)versions of PHP and I got different results:
RH7.3 with PHP 4.1.2: 2113879380
IRIX 6.5.11 with PHP 4.2.3: 2147483647
FC1 with PHP 4.3.6: 2113879380
RH7.3 with PHP 4.3.9: 2113879380
FC3 with PHP 4.3.10: -2147483648
FC4 with PHP 4.4.0: -2147483648
FC4 with PHP 5.0.5: -2147483648
So my question now, is it version or platform related?
I'd guess it depends on the compiler on the machine and (maybe) the libc version. Probably best to ask on the developer list about it.