MySQL distance function. Yes, just use PostGIS. Whatever.

dist_functions.sql

CREATE FUNCTION DISTANCE_MI( start_lat DOUBLE, start_lon DOUBLE, end_lat DOUBLE, end_lon DOUBLE )
RETURNS DOUBLE DETERMINISTIC
RETURN 3959 *
  acos(
    cos( radians(start_lat) )
    * cos( radians( end_lat ) )
    * cos(
      radians(end_lon) - radians(start_lon)
    )
    + sin( radians(start_lat) )
    * sin( radians(end_lat) )
  );
  
CREATE FUNCTION DISTANCE_KM( start_lat DOUBLE, start_lon DOUBLE, end_lat DOUBLE, end_lon DOUBLE )
RETURNS DOUBLE DETERMINISTIC
RETURN 6378 *
  acos(
    cos( radians(start_lat) )
    * cos( radians( end_lat ) )
    * cos(
      radians(end_lon) - radians(start_lon)
    )
    + sin( radians(start_lat) )
    * sin( radians(end_lat) )
  );