Most of the web by traffic or number of websites?

Largely both I'd imagine. Facebook, Slack, and Etsy all still run PHP (or Hack which is a PHP flavor) as far as I know. I'm sure there are a ton of other large web properties doing the same for their web tier.

All of the main adult industry properties run on php and that's a lot of traffic.

Hack is not a PHP flavour. It's a separate language that's inspired by PHP, with its own separate interpreter (HHVM).

Etsy still uses PHP, as does Vimeo and Wikimedia. A few other launched-in-early-oughts web properties rely heavily on it too (e.g. Flickr).