Google Map: is a lat/lng within a polygon?

Question

Given a pair of lat/lng values, how do I determine if the pair is within a polygon? I need to do this in PHP. I see that Google Maps API has a containsLocation

Answer 1

very big thanks to David Strachan and Darel Rex Finley

i want to share my php version, is slightly different beacause it takes the point as an array ([lat, lng]) and the polygon as an array of point ([[lat, lng],[lat, lng],...])

  function pointInPolygon($point, $polygon){//http://alienryderflex.com/polygon/
     $return = false;
     foreach($polygon as $k=>$p){
        if(!$k) $k_prev = count($polygon)-1;
        else $k_prev = $k-1;

        if(($p[1]< $point[1] && $polygon[$k_prev][1]>=$point[1] || $polygon[$k_prev][1]< $point[1] && $p[1]>=$point[1]) && ($p[0]<=$point[0] || $polygon[$k_prev][0]<=$point[0])){
           if($p[0]+($point[1]-$p[1])/($polygon[$k_prev][1]-$p[1])*($polygon[$k_prev][0]-$p[0])<$point[0]){
              $return = !$return;
           }
        }
     }
     return $return;
  }