Error while updating using yum -y update

I am getting this error constantly:"Package php-pear-1.8.0-1.el5.vm.noarch.rpm is not signed."

I assume, because of this error, i am also unable to update horde-3.2.1-2.el5.noarch.rpm

Yum says that the above two packages are set to be updated and downloads them but fails with the above cited error messsage.

Thanks in advance,

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