Mcrypt is installed, but error implies it's not.

php -v:
PHP 5.5.18 (cli) (built: Oct 20 2014 16:18:57)

php -m:
includes mcrypt in the list of extensions

var_dump(function_exists('mcrypt_encrypt')):
bool(true)

The link your provided is a tutorial for installing mcrypt on an earlier version of OS X ("Mountain Lion"). I'm running the latest version, "Yosemite", which is OS X 10.10.1.
My problem isn't installing mcrypt. Mcrypt is already installed in the version of PHP I'm running. That's verified when I run phpinfo(). It's listed as "enabled". It also displays in the list when I issue "php -m".