I'm not sure if this is a build or a documentation problem (or a little of both):  if you try to build PHP as an Apache DSO on a Red Hat Linux 6.1 install (with apache and apache-devel-1.3.9-8 rpms) and the standard MySQL rpms from www.mysql.net (3.22.30-1) and don't explicitly specify the path to mysql to configure, ie.:

./configure --with-apxs=/usr/sbin/apxs --with-mysql

then it still builds a libphp3.so, but when you try to start Apache with it, the httpd process refuses to start (no error messages or warnings, either).

However, if you specify "--with-mysql=/usr", then the libphp3.so builds and works just fine.

I know there have been other bug reports that say that you need to specify a directory for --with-mysql, but in this case if you don't, you don't get any errors from the build process (nor from Apache) - the libphp3.so gets built but it just doesn't work.  Which is pretty confusing when you try to debug the problem... :-0