To install a composer package globally, you run the usual require command, but with the addition of the global modifier. So to install PHPUnit, you would run:
$ composer global require phpunit/phpunit
$ composer global require phpunit/dbunit
$ composer global require phing/phing
$ composer global require phpdocumentor/phpdocumentor
$ composer global require sebastian/phpcpd
$ composer global require phploc/phploc