How do I Install an earlier version of Drupal 8 with composer because install fails on server with PHP 5.6?
Drupal 8 does not support PHP version 5. It requires a minimum PHP version of 7.0. Please see here for Drupal install requirements

Drupal Installation Requirements - Drupal.org

So you will have to upgrade PHP on your machine. Also PHP 5.0 is no longer supported and is not secure. So you might want to upgrade PHP on your server any way.

PHP Supported Versions - php.net