i compiled php with gd and all was well.
a few days later on i upgraded gd and restarted apache,
apache:
Starting httpd: Syntax error on line 250 of /etc/httpd/conf/httpd.conf:
Cannot load /usr/lib/apache/libphp4.so into server: /usr/lib/libgd.so.1: undefined symbol: png_set_dither

make output: could not find libgd.(a|so) anywhere in /usr/lib (although its there for sure)

to get php up and running again without any loss of time i decided to do a recompile without gd.
so i recompiled php: 
rm config.cache
./configure --with-apxs
make
make install

but apache still gives the same error when starting although gd isnt even compiled into php this time!!

this made me think.
so i decided to to try a few different versions of gd and kept getting exactly the same apache error everytime although gd was working fine for webalizer..

solution:
delete the whole source dir.
untar src again and compile again.

now it works.

so removing the config.cache doesnt give you a clean install at all, it seems to repeat the first install exactly everytime, although the INSTALL file says that removing the config.cache file should get you a fresh install.

maybe this isnt a bug but a feature, but i dont seem to remember php did this in previous versions....?