How to extend polygon by a certain distance in PHP/Mysql?

问题

I have a location defined by a polygon (e.g.(-8.87 41.86,-8.74 41.98,-8.63 42.06,-8.16 42.13 ...) ) and want to search for places from db within the polygon extended by a distance.

Google maps algorithm: http://www.geocodezip.com/v3_map-markers_ConvexHull_extend.asp

Any idea how to get extended polygon in PHP/MySQL in order to search for records from MySQL? I use ST_CONTAINS mysql function to search for records within an exact polygon.

来源：https://stackoverflow.com/questions/35848073/how-to-extend-polygon-by-a-certain-distance-in-php-mysql