Distance calculation between two zip codes(post codes) in php?

[qaz]

I am working on a website which requires distance calculation between two zip-codes. I have a database table which consists of zip-codes and their related latitudes and longitudes. I make use of this to calculate distance between the two places. But I have a problem that this gives me straight line distance and not driving distance - how can I find that?I'm using the following code\[code\] $lat1 = $this->deg_to_rad($lat1); $lon1 = $this->deg_to_rad($lon1); $lat2 = $this->deg_to_rad($lat2); $lon2 = $this->deg_to_rad($lon2); $delta_lat = $lat2 - $lat1; $delta_lon = $lon2 - $lon1; $temp = pow(sin($delta_lat/2.0),2) + cos($lat1) * cos($lat2) * pow(sin($delta_lon/2.0),2); $distance = 3956 * 2 * atan2(sqrt($temp),sqrt(1-$temp));\[/code\]