在PHP中使用透明度合并两个图像

这是我创建合成图像的代码：

$base = imagecreatefrompng('application/assets/images/vel1_bg.png');
    imagealphablending($base,true);
    list($baseWidth,$baseHeight,$type,$attr) = getimagesize('application/assets/images/vel1_bg.png');

    $user_board_items = $this->config->item('user_board_items');

    foreach($array as $key => $value){
        $item = imagecreatefrompng('application/assets/images/items/' . $user_board_items[$value[0]] . '.png');         
        imagealphablending($item,true);
        list($width,$height,$attr) = getimagesize('application/assets/images/items/'. $user_board_items[$value[0]] . '.png');

        imagecopymerge($base,$item,floor(($value[1] / 100) * $baseWidth),floor(($value[2] / 100) * $baseHeight),$width,100);
        imagedestroy($item);
    }

    //We have to capture the output buffer
    ob_start();
    imagepng($base);
    $baseimg = ob_get_clean();

这会生成如下图像：

我正在寻找更像这样的东西：

(注意如何表示透明部分)