php png水印不透明怎么办

/**
     * 为图片添加水印
     * @static public
     * @param string $source 原文件名
     * @param string $water  水印图片
     * @param string $$savename  添加水印后的图片名
     * @param string $alpha  水印的透明度
     * @return void
     */
    public function water($source, $water, $savename=null, $alpha=80 ,$w_pos = 9) {
        //检查文件是否存在
        if (!file_exists($source) || !file_exists($water))
            return false;
        //图片信息
        $sInfo = self::getImageInfo($source);
        $wInfo = self::getImageInfo($water);
        //如果图片小于水印图片，不生成图片
        if ($sInfo["width"] < $wInfo["width"] || $sInfo['height'] < $wInfo['height'])
            return false;
        //建立图像
        $sCreateFun = "imagecreatefrom" . $sInfo['type'];
        $sImage = $sCreateFun($source);
        
        $wCreateFun = "imagecreatefrom" . $wInfo['type'];
        $wImage = $wCreateFun($water); //$wImage
        
          
        //2.上色   //防止png透明背景变黑 
        $color=imagecolorallocate($wImage,255,255,255); 
        //3.设置透明 
        imagecolortransparent($wImage,$color); 
        imagefill($wImage,0,0,$color); 
        
        //设定图像的混色模式
        imagealphablending($wImage, true);
        //imageAlphaBlending($wImage, false);
        //imageSaveAlpha($wImage, true);
        //图像位置,默认为右下角右对齐
        //$posY = $sInfo["height"] - $wInfo["height"];
        //$posX = $sInfo["width"] - $wInfo["width"];
switch($w_pos) {
    case 1:
      $wx = 5;
      $wy = 5;
      break;
    case 2:
      $wx = ($sInfo["width"] - $wInfo["width"]) / 2;
      $wy = 0;
      break;
    case 3:
      $wx = $sInfo["width"] - $wInfo["width"];
      $wy = 0;
      break;
    case 4:
      $wx = 0;
      $wy = ($sInfo["height"] - $wInfo["height"]) / 2;
      break;
    case 5:
      $wx = ($sInfo["width"] - $wInfo["width"]) / 2;
      $wy = ($sInfo["height"] - $wInfo["height"]) / 2;
      break;
    case 6:
      $wx = $sInfo["width"] - $wInfo["width"];
   $wy = ($sInfo["height"] - $wInfo["height"]) / 2;
      break;
    case 7:
      $wx = 0;
      $wy = $sInfo["height"] - $wInfo["height"];
      break;
    case 8:
      $wx = ($sInfo["width"] - $wInfo["width"]) / 2;
      $wy = $sInfo["height"] - $wInfo["height"];
      break;
    case 9:
      $wx = $sInfo["width"] - $wInfo["width"];
      $wy = $sInfo["height"] - $wInfo["height"];
      break;
    case 10:
      $wx = rand(0,($sInfo["width"] - $wInfo["width"]));
      $wy = rand(0,($sInfo["height"] - $wInfo["height"]));
      break;       
    default:
      $wx = $sInfo["width"] - $wInfo["width"];
      $wy = $sInfo["height"] - $wInfo["height"];
      break;
  }
        //生成混合图像
        imagecopymerge($sImage, $wImage, $wx, $wy, 0, 0, $wInfo['width'], $wInfo['height'], $alpha);
        //输出图像
        $ImageFun = 'Image' . $sInfo['type'];
        //如果没有给出保存文件名，默认为原图像名
        if (!$savename) {
            $savename = $source;
            @unlink($source);
        }
        //保存图像
        $ImageFun($sImage, $savename);
        imagedestroy($sImage);
    }

 /**
     * 取得图像信息
     * @static
     * @access public
     * @param string $image 图像文件名
     * @return mixed
     */
    static function getImageInfo($img) {
        $imageInfo = getimagesize($img);
        if ($imageInfo !== false) {
            $imageType = strtolower(substr(image_type_to_extension($imageInfo[2]), 1));
            $imageSize = filesize($img);
            $info = array(
                "width" => $imageInfo[0],
                "height" => $imageInfo[1],
                "type" => $imageType,
                "size" => $imageSize,
                "mime" => $imageInfo['mime']
            );
            return $info;
        } else {
            return false;
        }
    }

//2.上色   //防止png透明背景变黑 
        $color=imagecolorallocate($wImage,255,255,255); 
        //3.设置透明 
        imagecolortransparent($wImage,$color); 
        imagefill($wImage,0,0,$color);