Installing PHP Composer in Ubuntu

$ curl -s http://getcomposer.org/installer | php
$ sudo mv composer.phar /usr/local/bin/composer.phar

And if you're even more lazy, like me, you can create an alias:
$ alias composer='/usr/local/bin/composer.phar'

This way you can invoke composer with just composer

Update: Composer now offers another method as well on the website:
$ curl -sS https://getcomposer.org/installer | php
$ (sudo) mv composer.phar /usr/local/bin/composer