Quoted from
http://php.net/manual/en/language.oop5.constants.php:
"The special ::class constant are available as of PHP 5.5.0, and allows for fully qualified class name resolution at compile, this is useful for namespaced classes"
But you explain php 5.6 is already included in the dokuwiki on stick package, so it should be supported..
Are you sure that you use this recent version in your usb setup?