Everybody, who tried to get composer running on a shared web hosting package at IONOS will have faced the same problem: the installed PHP-CLI version is pretty old. Something around PHP 4.4. But what you'll probably didn't know: IONOS has newer PHP-CLI versions installed in your users directory.

So it's a little bit difficult, but you can use composer at IONOS. Here is how.

# install composer through php-cli
$ curl -sS https://getcomposer.org/installer | /usr/bin/php7.3-cli

# run composer and test if it's working
$ /usr/bin/php7.3-cli composer.phar

# update composer to latest version
$ /usr/bin/php7.3-cli composer.phar selfupdate

# install project dependencies
$ /usr/bin/php7.3-cli composer.phar install