Composer is an extremely commonly used PHP package manager. You will need/want it if you're working with PHP.
Run the following to install composer and make it available to the entire system. This requires root access. You can run these commands from any folder as they will clean up after themselves:
sudo su php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" php composer-setup.php mv composer.phar /usr/local/bin/composer rm composer-setup.php
You can now access composer from anywhere!