It works fine with Varnish 4, open source version. Varnish Plus is not required.

Thank you Adrian Hernandez http://php.net/manual/en/varnish.requirements.php#115883
Yes this manual should be updated to make note of this. 
may I ask simply why this requires Plus? I just lost hours implementing this only to see the extension refuses to load on my prod server :\

It should be mention that Varnish PLUS is required.