Calculate distance based on GPS location

Hello, At the moment i'm developing a comparison-related website where visitors can fill in their zipcode, and based on that zipcode (And Latitude/Longtitude which is being received via a partner website, doesn't need to be developped) they need their offer-request to be sended towards companies which are registered with us. The following needs to be made: - Companies who register, needs to have the possibility to choose if they want to receive offers from a certain province, or from X km's from their own ZIPcode (Lat/Lon via the partner coupled). - When a visitor makes an offer-request at the website, this offer needs to be sended to companies which are in the related province, or in the KM-range which is filled in. (With a max. of 6 offers, and it needs to be even over all companies in the database, not that only the first 6 ones in the database for the certain province/km receive the offers) Kind regards, Yannick