For God-knows-what reason, the libmcrypt and libmhash aren't included in the default repository for CentOS 6 or Red Hat Enterprise Linux 6. So you'll run into the following when trying to compile PHP.
configure: error: mcrypt.h not found. Please reinstall libmcrypt.
And you can't install it.
# yum install libmcrypt-devel
No package libmcrypt-devel available.
Error: Nothing to do
The (only) solution so far: grab the packages from the EPEL repository. Download the RPM (it's same RPM for i386, i686 and x86_64) and install it.
# rpm -ivh “http://dl.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm”
And you can install it as "yum install libmcrypt-devel" again. Or libmhash, just the same.