mcrypt_module_open() warning !-Collection of common programming errors

The mcrypt module is enabled in your php.ini, otherwise you would get an error message along the lines Call to undefined function.

What it indicates is that the requested cipher method is not available. You have either a typo, or one of the ciphers is not compiled into your version of mcrypt.so.

 mcrypt_module_open('rijndael-256', '', 'ofb', '');   // works

 mcrypt_module_open('wrong', '', '', '');   // generates your error

The manual page and the extra parameters indicate that the libmcrypt on your system might depend on shared modules itself. So you might want to search for another version. Try a MAMP update, or use the pro version that exists for that PHP distribution.