Don't use obsolete/unsupported releases - 5.8 is the current release. See the CentOS 5.8 Release Notes
for details. By not updating you are implicitly accepting that you will live with numerous bugs and security issues (and associated known exploits) that have subsequently been fixed.
I don't see any php-mhash packages for php 5.3 anywhere. The 5.1 version you are trying to install is incompatible.
You could try following How to Rebuild a Source RPM
to build a compatible version, but as there is not even one around for CentOS-6 there may be a serious problem that precludes using it.