The phpcs-symfony project builds a PHP_CodeSniffer standard for the symfony framework and projects built using the symony framework.
Obviously, PHP_CodeSniffer has to be installed.
sudo pear install PHP_CodeSniffer-1.3.0RC1
If you want to install the standard you have to symlink this project root folder into the Standard folder where PHP_CodeSniffer is installed. Usually this is /usr/share/php/PHP/CodeSniffer/Standard if PHP_Codesniffer is installed via PEAR.
sudo ln -s /folder/to/git/folder/phpcs-symfony/ /usr/share/php/PHP/CodeSniffer/Standards/Symfony
After installing the standard you are able to use it with the phpcs standard, you can now go in Symfony root folder and execute :
phpcs --standard=Symfony src/