Correct command should be:
# /opt/plesk/php/5.6/bin/php -v
PHP 5.6.22 (cli) (built: Jun 7 2016 10:33:10)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
with the ionCube PHP Loader (enabled) + Intrusion Protection from ioncube24.com (unconfigured) v5.0.18, Copyright (c) 2002-2015, by ionCube Ltd.
with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2016, by Zend Technologies