title: Require version of package depending on installed PHP version?
body: <p>Does Composer provide a way to require a particular version of a package depending on the installed PHP version?</p>
  
  <p>E.g. if PHP 7.3 is the installed PHP version when I run <code>composer install</code> then PHPUnit version 8 should be installed. If I changed my environment to use PHP 5.6 and ran <code>composer install</code> then PHPUnit 5 should be installed.</p>