How do I use it in a system with 2 PHP-versions? On my server there are two versions of PHP: 5.4 and 5.6. Using the shell PHP 5.4 is the standard and it can't be changed because of PLESK/ODIN. The websites I'm administering are using PHP 5.6 via PLESK, so Drupal 8 works fine. Drush doesn't,  because it uses the shell.

I have installed Drush with the following commands: 

php drush.phar core-status
mv drush.phar /usr/local/bin/drush
drush init


How can I change this to use PHP 5.6 in /opt/plesk/5.6?
Drush has an environment variable DRUSH_PHP to set the php executable. You can set it in your .bashrc or directly on the command line:

env DRUSH_PHP=/opt/plesk/5.6/php drush status