Are you trying to install Symfony version 2.3 but get the following error message?
symfony/icu v1.2.0-RC1 requires lib-icu >=4.4 -> the requested linked library icu has the wrong version installed or is missing from your system, make sure to have the extension providing it.
This comes from a requirement in the new Icu component but it is easily fixed. Just install the intl extension for PHP. In Ubuntu with Apt:
$ sudo apt-get install php5-intl
Or if you are using Mac:
(PHP 5.4)
$ brew install php54-intl
(PHP 5.5)
$ brew tap josegonzalez/php
$ brew tap homebrew/dupes
$ brew install josegonzalez/php/php55-intl
And then you are good to go!