>从原始图像开始 – $img
>将该图像复制到png – $copy
>在圆圈/椭圆中创建所需区域的蒙版png图像(“magicpink”图像上带有黑色形状,黑色设置为Alpha透明度的颜色) – $mask
>将$mask复制到$copy的顶部,保持Alpha透明度
>在$copy上更改您需要的内容
>将$copy复制回$img,保持Alpha透明度

// 1. Start with the original image  
    $img = imagecreatefromjpeg("./original.jpg");  
    $img_magicpink = imagecolorallocatealpha($img,255,127);  
    //imagecolortransparent($img,$img_magicpink);  

    // (Get its dimensions for copying)  
    list($w,$h) = getimagesize("./original.jpg");  

    // 2. Create the first copy  
    $copy = imagecreatefromjpeg("./original.jpg");  
    imagealphablending($copy,true);  

    $copy_magicpink = imagecolorallocate($copy,255);  
    imagecolortransparent($copy,$copy_magicpink);  

    // 3. Create the mask  
    $mask = imagecreatetruecolor($w,$h);  
    imagealphablending($mask,true);  

    // 3-1. Set the masking colours  
    $mask_black = imagecolorallocate($mask,0);  
    $mask_magicpink = imagecolorallocate($mask,255);  
    imagecolortransparent($mask,$mask_black);  
    imagefill($mask,$mask_magicpink);  

    // 3-2. Draw the circle for the mask  
    $circle_x = $w/2;  
    $circle_y = $h/2;  
    $circle_w = 150;  
    $circle_h = 150;  
    imagefilledellipse($mask,$circle_x,$circle_y,$circle_w,$circle_h,$mask_black);  

    // 4. Copy the mask over the top of the copied image,and apply the mask as an alpha layer  
    imagecopymerge($copy,$mask,$w,$h,100);  


    // 5. Do what you need to do to the image area  
    // My example is turning the original image gray and leaving the masked area as colour  
    $x = imagesx($img);  
    $y = imagesy($img);  
    $gray = imagecreatetruecolor($x,$y);  
    imagecolorallocate($gray,0);  
    for ($i = 0; $i > 16) & 0xFF;  
        $g = ($rgb >> 8) & 0xFF;  
        $b = $rgb & 0xFF;  
         //for gray mode $r = $g = $b  
        $color = max(array($r,$g,$b));  
        $gray_color = imagecolorexact($img,$color,$color);  
        imagesetpixel($gray,$i,$j,$gray_color);  
      }  
    }  

    // 6. Merge the copy with the origianl - maintaining alpha  
    imagecopymergegray($gray,$copy,100);  
    imagealphablending($gray,true);  
    imagecolortransparent($gray,$mask_magicpink);  

    header('Content-Type: image/png');  
    imagepng($gray);  
    imagedestroy($gray);