Polygon::getCentroid PHP Method

    public function getCentroid()
    {
        $return = null;
        if (count($this->points) >= 3) {
            $centroid = new Point(0, 0);
            $signedArea = 0;
            $x0 = $y0 = $x1 = $y1 = 0;
            $a = 0;
            // Partial signed area
            $vertices = $this->points;
            $vertices[] = $vertices[0];
            // For all vertices except last
            for ($i = 0; $i < count($vertices) - 1; $i++) {
                $x0 = $vertices[$i]->x;
                $y0 = $vertices[$i]->y;
                $x1 = $vertices[$i + 1]->x;
                $y1 = $vertices[$i + 1]->y;
                $a = $x0 * $y1 - $x1 * $y0;
                $signedArea += $a;
                $centroid->x += ($x0 + $x1) * $a;
                $centroid->y += ($y0 + $y1) * $a;
            }
            $signedArea *= 0.5;
            $centroid->x /= 6 * $signedArea;
            $centroid->y /= 6 * $signedArea;
            $return = $centroid;
        }
        return $return;
    }