PHP error: "The zip extension and unzip command are both missing, skipping."

When I run a `composer update` I get this error message: Loading composer repositories with package information Updating dependencies (including require-dev) Failed to download psr/log from dist: The zip extension and unzip command are both missing, skipping. The php.ini used by your command-line PHP is: /etc/php/7.0/cli/php.ini Now trying to download from source What do I need to do to enable the zip and unzip commands so that composer can download dependencies?