Composer requires PHP 5.3.2+ to run. A few sensitive php settings and compile flags are also required, but when using the installer you will be warned about any incompatibilities.
First, go to the /tmp directory
Download the composer.phar file
curl -sS https://getcomposer.org/installer | php
Move it to
mv composer.phar /usr/local/bin/composer
now you should be able to use the command