SHA384 is not supported by your openssl extension, could not verify the phar file integrity

Remove Your older versions and install the latest version of Composer,

Remove Your older version:
sudo rm -f /usr/local/bin/composer

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

Move the composer.phar file:
sudo mv composer.phar /usr/local/bin/composer