In order to compile php on 64bit platforms make sure that your configure command contains:
--with-libdir=lib64
--libdir=/usr/lib64

When I compiled php --with-curl on Centos 5.2 64bit I also encountered a strange error in the configure command:

checking for curl_easy_perform in -lcurl... no

It seems that it is not correctly detecting a package version. I only had libidn-devel i386 in the standard setup and after
yum install libidn-devel

in another instance based on the same configure error I could see the following issue in config.log