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Howdy,
It doesn't look like RHEL/CentOS includes the mcrypt PHP module with their distribution.
You may need to get it from another source, such as the EPEL repository.
We haven't tested this, but I do see that module here in this repository:
http://cosmos.cites.illinois.edu/pub/centos/7.0.1406/os/x86_64/Packages/