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. }