After upgrading PHP5 from packages I built on a FreeBSD 6.2 machine on a 6.1 machine, Apache wouldn't start, giving the following error:

# apachectl start
Syntax error on line 273 of /usr/local/etc/apache2/httpd.conf: Cannot load /usr/local/libexec/apache2/libphp5.so into server:
/usr/local/libexec/apache2/libphp5.so: Undefined symbol "__res_ninit"

I discovered that this symbol, "__res_ninit", was added to /lib/libc.so.6 sometime between 6.1 and 6.2. To get around this problem I made new packages on a 6.1 machine. It would also be possible to make a 6.1 chroot and build the packages there.