Accessing environment sensor data from a web page (HTML/JS/PHP) Is it possible to fetch the temperature info on the device from a web page using html/javascript/php?

Or would an app be required to do this?
No, not with HTML/JavaScript/PHP.

HTML is just plain information display. It can't "do" anything by itself.

JavaScript doesn't has access to the device sensors (luckily).

PHP is server side...

You could do it maybe with Flash/Java Plugins but those are unsafe and don't work (as far as I know) on mobile devices.

So yes, it would require an app. What you could do is to try to find the location of the device (thats something you can do with JavaScript/PHP) and look up the temperature of the location returned. But the location requests often don't work completely (e.g. they return very vague locations, or the location of the provider), so I would recommend you to just write an app.