This installer checks if you have Mcrypt installed.
If Mcrypt exists and functions, the installer will leave your installation as-is.
If Mcrypt does not exist/function, its installed for you.
The Mcrypt libraries are installed to
/usr/local/lib/libmcrypt
PHP modules for both OS X 10.8 and 10.9 are installed here
/usr/lib/php/extensions/no-debug-non-zts-20090626
/usr/lib/php/extensions/no-debug-non-zts-20100525
Your system will use the correct version
Then the installer will add to /etc/php.ini
extension=mcrypt.so
The topicdesk Mcrypt installer is a free download.
Download here
Source: topicdesk