$point=[
      'lng'=>121.427417,
      'lat'=>31.20357
  ];
  $arr=[
      [
          'lng'=>121.23036,
          'lat'=>31.218609
      ],
      [
          'lng'=>121.233666,
          'lat'=>31.210579
      ],
      [
          'lng'=>121.247177,
          'lat'=>31.206749
      ],
      [
          'lng'=>121.276353,
          'lat'=>31.190811
      ],
      [
          'lng'=>121.267442,
          'lat'=>31.237383
      ],
  ];
  
  $a= is_point_in_polygon($point, $arr);
  var_dump($a);


/**
 * 判断一个坐标是否在一个多边形内（由多个坐标围成的）
 * 基本思想是利用射线法，计算射线与多边形各边的交点，如果是偶数，则点在多边形外，否则
 * 在多边形内。还会考虑一些特殊情况，如点在多边形顶点上，点在多边形边上等特殊情况。
 * @param $point 指定点坐标
 * @param $pts 多边形坐标 顺时针方向
 */
function is_point_in_polygon($point, $pts) {
    $N = count($pts);
    $boundOrVertex = true; //如果点位于多边形的顶点或边上，也算做点在多边形内，直接返回true
    $intersectCount = 0;//cross points count of x 
    $precision = 2e-10; //浮点类型计算时候与0比较时候的容差
    $p1 = 0;//neighbour bound vertices
    $p2 = 0;
    $p = $point; //测试点
 
    $p1 = $pts[0];//left vertex        
    for ($i = 1; $i <= $N; ++$i) {//check all rays
        // dump($p1);
        if ($p['lng'] == $p1['lng'] && $p['lat'] == $p1['lat']) {
            return $boundOrVertex;//p is an vertex
        }
         
        $p2 = $pts[$i % $N];//right vertex            
        if ($p['lat'] < min($p1['lat'], $p2['lat']) || $p['lat'] > max($p1['lat'], $p2['lat'])) {//ray is outside of our interests
            $p1 = $p2; 
            continue;//next ray left point
        }
         
        if ($p['lat'] > min($p1['lat'], $p2['lat']) && $p['lat'] < max($p1['lat'], $p2['lat'])) {//ray is crossing over by the algorithm (common part of)
            if($p['lng'] <= max($p1['lng'], $p2['lng'])){//x is before of ray
                if ($p1['lat'] == $p2['lat'] && $p['lng'] >= min($p1['lng'], $p2['lng'])) {//overlies on a horizontal ray
                    return $boundOrVertex;
                }
                 
                if ($p1['lng'] == $p2['lng']) {//ray is vertical                        
                    if ($p1['lng'] == $p['lng']) {//overlies on a vertical ray
                        return $boundOrVertex;
                    } else {//before ray
                        ++$intersectCount;
                    }
                } else {//cross point on the left side
                    $xinters = ($p['lat'] - $p1['lat']) * ($p2['lng'] - $p1['lng']) / ($p2['lat'] - $p1['lat']) + $p1['lng'];//cross point of lng
                    if (abs($p['lng'] - $xinters) < $precision) {//overlies on a ray
                        return $boundOrVertex;
                    }
                     
                    if ($p['lng'] < $xinters) {//before ray
                        ++$intersectCount;
                    } 
                }
            }
        } else {//special case when ray is crossing through the vertex
            if ($p['lat'] == $p2['lat'] && $p['lng'] <= $p2['lng']) {//p crossing over p2
                $p3 = $pts[($i+1) % $N]; //next vertex
                if ($p['lat'] >= min($p1['lat'], $p3['lat']) && $p['lat'] <= max($p1['lat'], $p3['lat'])) { //p.lat lies between p1.lat & p3.lat
                    ++$intersectCount;
                } else {
                    $intersectCount += 2;
                }
            }
        }
        $p1 = $p2;//next ray left point
    }
 
    if ($intersectCount % 2 == 0) {//偶数在多边形外
        return false;
    } else { //奇数在多边形内
        return true;
    }
}