How to use another php version for drush 9?
Try editing both the .bash_rc and .bash_profile files so that you include the directory where the version of PHP you want is FIRST (before existing PATH):

(add this line or adjust if existing):

export PATH="/opt/php70/bin:$PATH"

As a last ditch effort, you can set the env variable drush looks for as well in these files:

DRUSH_PHP=/opt/php70/bin/php