This has been tested in Rebecca and Rosa.
Open the terminal in your $HOME directory. The $HOME directory is typically at /home/yourname. You can easily get to it from anywhere in the terminal just by typing cd and pressing enter.
1. Enter the following:
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
You may get a message that you need to install php5-cli using apt-get. If needed do it by typing: