The default php53 packages set on CentOS repo does not contain mcrypt support. To get this library working for your web server you can take one of two ways. Either build php53 mcrypt.so extension yourself, there is a short guide for that. Or go the lazy way:
1) Add EPEL repo to your repositories list.
2) Install the mcrypt module:
3) Edit your php.ini, and add extension support
4) Restart web server.
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