When installing the memcache module for Apache/PHP with a non-standard location it seems like there is no way to define the location when configuring for build. You'll get an error complaining about being unable to find php_session.h. I modified configure to try the PREFIX location too and that seems to have worked fine.
if test -f "${prefix}/include/php/ext/session/php_session.h"; then
session_inc_path="${prefix}/include/php"
elif test -f "$abs_srcdir/include/php/ext/session/php_session.h"; then
session_inc_path="$abs_srcdir/include/php"
elif test -f "$abs_srcdir/ext/session/php_session.h"; then
session_inc_path="$abs_srcdir"
elif test -f "$phpincludedir/ext/session/php_session.h"; then
session_inc_path="$phpincludedir"
fi