I am trying to install composer by that way and get some errors.
Step 1: Ensure the Curl dependency is installed.

sudo apt-get -y install curl

Step 2: Download the Composer installer and pass it to php.

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

My error in this step:

The program 'php' can be found in the following packages:
* php7.0-cli
* hhvm
Try: sudo apt install <selected package>
curl: (23) Failed writing body (0 != 16133)