最简单的方案：

curl -sS https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/composer

安装完后最好更新一下：

composer self-update

下面是老方案，可以不看了....

Step 1:

The recommended path, not needed.

cd /usr/local/bin

Step 2:

install

 sudo curl -s https://getcomposer.org/installer | sudo php

Step 3:

Add execute PErmissions.

sudo chmod a+x composer.phar

Step 4:

This step is not necessary. If needed, you can use this script to update Composer.

sudo composer.phar self-update