You can install the composer by running following commands in your terminal.
sudo php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
sudo php -r "if (hash_file('SHA384', 'composer-setup.php') === '544e09ee996cdf60ece3804abc52599c22b1f40f4323403c44d44fdfdd586475ca9813a858088ffbc1f233e9b180f061') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
sudo php composer-setup.php
sudo php -r "unlink('composer-setup.php');"