PIR sensor: that's just a basic on/off signal. It doesn't count people, it just informs you of the presence of something warm that's moving. That's what they do. I have no idea how you intend to relate this to anything resembling the number of people, unless you have someone at the door that makes sure only one person at a time can pass through, and that there is at least 1-2 seconds between people passing to allow the PIR to deactivate (usually 1-2 seconds is the minimum time it'll stay active upon detection of movement).
For the html part you could use an ESP8266 as simple web server, it serving the page with the counter to you. I don't know enough of javascript to know whether you can have it update automatically, but you can dynamically create the page with the latest number included and re-request it regularly.