How to fix the «The "https://repo.magento.com/packages.json" file could not be downloaded: allow_url_fopen must be enabled in php.ini» issue?

Using php -d allow_url_fopen=on allows a person to override php.ini. Then the full path to composer needs to be explicitly added…

Example:
# php -d allow_url_fopen=on /opt/bin/composer update

Adding this here for next person who Google’s their way here.