Since you're on Windows, an alternative to the exec() approach would be to use COM. But then you would have to find a COM object to use to get the data (otherwise, you'd have to create your own COM object). PHP, by itself, can't get the data. But its very possible to have it work with another program or script to get the data you want.
I might recommend having a process that runs every X minutes. Have it read the temperature and have it save the data to a text file (the format would be up to you). Then have PHP fetch the data from the file (PHP is really good at doing this) whenever you need to. But this depends on how frequent you need to update the data and how many people will be requesting this data.
