php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '756890a4488ce9024fc62c56153228907f1545c228516cbf63f885e036d37e9a59d27d63f46af1d4d07ee0f76181c7d3') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"

#move composer.phar, so you can call composer command
sudo mv composer.phar /usr/local/bin/composer

#update composer to latest version
composer self-update

cd ~
curl -sS https://getcomposer.org/installer | sudo php
sudo mv composer.phar /usr/local/bin/composer

Easiest, safest, verified tutorial

https://www.digitalocean.com/community/tutorials/how-to-install-composer-on-ubuntu-20-04-quickstart

# download composer.phar to /usr/bin and symlink composer to composer.phar
pushd /usr/bin >/dev/null &&
sudo wget -O composer.phar https://getcomposer.org/composer-stable.phar &&
sudo chmod 755 composer.phar &&
sudo ln -fs composer.phar composer &&
popd >/dev/null