Bagaimana untuk menetapkan gambar kepada gambar bulat dalam php

imagecolorat
imagesetpixel

(x-a)*(x-a)+(y-b)*(y-b)<r2

function yuan_img($imgpath) {
        $ext     = pathinfo($imgpath);
        $src_img = null;
        switch ($ext['extension']) {
        case 'jpg':
            $src_img = imagecreatefromjpeg($imgpath);
            break;
        case 'png':
            $src_img = imagecreatefrompng($imgpath);
            break;
        }
        $wh  = getimagesize($imgpath);
        $w   = $wh[0];
        $h   = $wh[1];
        $w   = min($w, $h);
        $h   = $w;
        $img = imagecreatetruecolor($w, $h);
        //这一句一定要有
        imagesavealpha($img, true);
        //拾取一个完全透明的颜色,最后一个参数127为全透明
        $bg = imagecolorallocatealpha($img, 255, 255, 255, 127);
        imagefill($img, 0, 0, $bg);
        $r   = $w / 2; //圆半径
        $y_x = $r; //圆心X坐标
        $y_y = $r; //圆心Y坐标
        for ($x = 0; $x < $w; $x++) {
            for ($y = 0; $y < $h; $y++) {
                $rgbColor = imagecolorat($src_img, $x, $y);
                if (((($x - $r) * ($x - $r) + ($y - $r) * ($y - $r)) < ($r * $r))) {
                    imagesetpixel($img, $x, $y, $rgbColor);
                }
            }
        }
        return $img;
    }