You can install Mcrypt from the PHP Source Tree as a module if you choose.
You first need to ensure you have libmcrypt, libmcrypt-devel, and mcrypt installed, then do:
# cd php-5.x.x/ext/mcrypt
# make && make install
Enable the module by adding: 'extension=mcrypt.so' to PHP.ini.
Very handy if you need to install a single module and you may have installed PHP via RPM, but don't wish to recompile your whole PHP install.