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
ubuntu
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