png.h not found, something's going wrong in my PHP installation

I found a thread a couple of weeks ago where someone had to create symbolic links in order to get their install of PHP to work because PHP wasn't able to find png.h. I am having the same problem. I made no changes to the installation of libpng, installed it in its default location and PHP cannot find png.h, no matter what I set the dir= flag to. Here is my ./configure: ./configure --with-mysql --with-apxs=/www/bin/apxs --with-xml --enable-ftp --with-gd --enable-magic-quotes --with-jpeg-dir=/usr --with-mcrypt --with-png-dir=/usr --enable-sockets --enable-track-vars --enable-versioning --with-zlib --enable-bcmath --enable-calendar --with-xpm-dir=/usr/X11R6 --enable-memory-limit png.h on my system is located in /usr/lib