I am using the latest version of Ubuntu and used the instructions for installing phalcon on Ubuntu.
apt-get install php5-phalcon
It seems to install OK, but I get the following warning when I restart Apache2. (Apache version....Server version: Apache/2.4.7 (Ubuntu) Server built: Jul 22 2014 14:36:38)
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php5/20121212/phalcon.so' - /usr/lib/php5/20121212/phalcon.so: undefined symbol: php_pdo_get_dbh_ce in Unknown on line 0