Calculate the Distance Between Two Coordinates (latitude, longitude)

1. function distance($lat1, $lng1, $lat2, $lng2, $miles = true)
2. {
3. $pi80 = M_PI / 180;
4. $lat1 *= $pi80;
5. $lng1 *= $pi80;
6. $lat2 *= $pi80;
7. $lng2 *= $pi80;
8.  
9. $r = 6372.797; // mean radius of Earth in km
10. $dlat = $lat2 - $lat1;
11. $dlng = $lng2 - $lng1;
12. $a = sin($dlat / 2) * sin($dlat / 2) + cos($lat1) * cos($lat2) * sin($dlng / 2) * sin($dlng / 2);
13. $c = 2 * atan2(sqrt($a), sqrt(1 - $a));
14. $km = $r * $c;
15.  
16. return ($miles ? ($km * 0.621371192) : $km);
17. }