sudo apt update
sudo apt install php-cli unzip
cd ~
curl -sS https://getcomposer.org/installer -o composer-setup.php

HASH=`curl -sS https://composer.github.io/installer.sig`
echo $HASH
php -r "if (hash_file('SHA384', 'composer-setup.php') === '$HASH') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer
composer

apt search mbstring
sudo apt install php-mbstring

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '8a6138e2a05a8c28539c9f0fb361159823655d7ad2deecb371b04a83966c61223adc522b0189079e3e9e277cd72b8897') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"

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

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '906a84df04cea2aa72f40b5f787e49f22d4c2f19492ac310e8cba5b96ac8b64115ac402c8cd292b8a03482574915d1a8') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
sudo php composer-setup.php --install-dir=/bin --filename=composer
php -r "unlink('composer-setup.php');"

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"

Teniendo composer instaldo, correr 
composer self-update --2