使用GD库做图片水印

1. png图片作为水印加到其他类型图片后，背景变黑色 
   原因： imagecopy函数拷贝时可以保留png图像的原透明信息，而imagecopymerge却不支持图片的本身的透明拷贝。 
   然后直接上代码：

   /*
       * 功能:PHP图片水印 (水印支持图片或文字)
       * 参数:
       * $groundImage 背景图片,即需要加水印的图片,暂只支持GIF,JPG,PNG格式;
       * $waterPos 水印位置,有10种状态,0为随机位置;
       * 1为顶端居左,2为顶端居中,3为顶端居右;
       * 4为中部居左,5为中部居中,6为中部居右;
       * 7为底端居左,8为底端居中,9为底端居右;
       * $waterImage 图片水印,即作为水印的图片,暂只支持GIF,JPG,PNG格式;
       * $waterText 文字水印,即把文字作为为水印,支持ASCII码,不支持中文;
       * $textFont 文字大小,值为1、2、3、4或5,默认为5;
       * $textColor 文字颜色,值为十六进制颜色值,默认为#FF0000(红色);
       *
       * 注意:Support GD 2.0,Support FreeType、GIF Read、GIF Create、JPG 、PNG
       * $waterImage 和 $waterText 最好不要同时使用,选其中之一即可,优先使用 $waterImage.
       * 当$waterImage有效时,参数$waterString、$stringFont、$stringColor均不生效.
       * 加水印后的图片的文件名和 $groundImage 一样.
   
       */
       public static function FoximageWaterMark($groundImage,$waterPos=0,$waterImage="",$waterText="",$textFont=5,$textColor="#595959")
       {
           $isWaterImage = FALSE;
           $formatMsg = "暂不支持该文件格式,请用图片处理软件将图片转换为GIF、JPG、PNG格式.";
           //读取水印文件
           if(!empty($waterImage) && file_exists($waterImage)) {
               $isWaterImage = TRUE;
               $water_info = getimagesize($waterImage);
               $water_w = $water_info[0];//取得水印图片的宽
               $water_h = $water_info[1];//取得水印图片的高
               switch($water_info[2])//取得背景图片的格式
               {
                   case 1:$water_im = imagecreatefromgif($waterImage);break;
                   case 2:$water_im = imagecreatefromjpeg($waterImage);break;
                   case 3:$water_im = imagecreatefrompng($waterImage);break;
                   default:die($formatMsg);
               }
           }
           //读取背景图片
           if(!empty($groundImage) && file_exists($groundImage)){
               $ground_info = getimagesize($groundImage);
               $ground_w = $ground_info[0];//取得背景图片的宽
               $ground_h = $ground_info[1];//取得背景图片的高
               switch($ground_info[2])//取得背景图片的格式
               {
                   case 1:$ground_im = imagecreatefromgif($groundImage);break;
                   case 2:$ground_im = imagecreatefromjpeg($groundImage);break;
                   case 3:$ground_im = imagecreatefrompng($groundImage);break;
                   default:die($formatMsg);
               }
           }
           else{
               die("需要加水印的图片不存在!");
           }
           //水印位置
           if($isWaterImage)//图片水印
           {
               $w = $water_w;
               $h = $water_h;
               $label = "图片的";
           }
           else//文字水印
           {
             $temp = imagettfbbox(ceil($textFont*5.5),0,"0",$waterText);//取得使用 TrueType 字体的文本的范围
             $w = $temp[2] - $temp[6];
             $h = $temp[3] - $temp[7];
             unset($temp);
             $label = "文字区域";
           }
         if( ($ground_w<$w) || ($ground_h<$h) )
         {
             echo "需要加水印的图片的长度或宽度比水印".$label."还小,无法生成水印!";
             return;
         }
           switch($waterPos)
           {
               case 0://随机
                   $posX = rand(0,($ground_w - $w));
                   $posY = rand(0,($ground_h - $h));
                   break;
               case 1://1为顶端居左
                   $posX = 0;
                   $posY = 0;
                   break;
               case 2://2为顶端居中
                   $posX = ($ground_w - $w) / 2;
                   $posY = 0;
                   break;
               case 3://3为顶端居右
                   $posX = $ground_w - $w;
                   $posY = 0;
                   break;
               case 4://4为中部居左
                   $posX = 0;
                   $posY = ($ground_h - $h) / 2;
                   break;
               case 5://5为中部居中
                   $posX = ($ground_w - $w) / 2;
                   $posY = ($ground_h - $h) / 2;
                   break;
               case 6://6为中部居右
                   $posX = $ground_w - $w;
                   $posY = ($ground_h - $h) / 2;
                   break;
               case 7://7为底端居左
                   $posX = 0;
                   $posY = $ground_h - $h;
                   break;
               case 8://8为底端居中
                   $posX = ($ground_w - $w) / 2;
                   $posY = $ground_h - $h;
                   break;
               case 9://9为底端居右
                   $posX = $ground_w - $w;
                   $posY = $ground_h - $h;
                   break;
               default://随机
                   $posX = rand(0,($ground_w - $w));
                   $posY = rand(0,($ground_h - $h));
                   break;
           }
           //设定图像的混色模式
           imagealphablending($ground_im,true);
           if($isWaterImage)//图片水印
           {
               // imagecopy函数拷贝时可以保留png图像的原透明信息，而imagecopymerge却不支持图片的本身的透明拷贝
               imagecopy($ground_im, $water_im, $posX, $posY, 0, 0, $water_w, $water_h);
   //            imagecopymerge($ground_im, $water_im, $posX, $posY, 0, 0, $water_w,$water_h,20);//拷贝水印到目标文件
           }
           else//文字水印
           {
               if( !empty($textColor) && (strlen($textColor)==7) )
               {
                   $R = hexdec(substr($textColor,1,2));
                   $G = hexdec(substr($textColor,3,2));
                   $B = hexdec(substr($textColor,5));
               }
               else
               {
                   die("水印文字颜色格式不正确!");
               }
   
            // imagettftext ($ground_im, 20,0, 20, 30, imagecolorallocate($ground_im, $R, $G, $B),base_url()."/water.ttf","麦言社区");
               imagestring ( $ground_im, $textFont, $posX, $posY, $waterText, imagecolorallocatealpha($ground_im, $R, $G, $B,100));
           }
           //生成水印后的图片
           @unlink($groundImage);
           switch($ground_info[2])//取得背景图片的格式
           {
               case 1:imagegif($ground_im,$groundImage);break;
               case 2:imagejpeg($ground_im,$groundImage);break;
               case 3:imagepng($ground_im,$groundImage);break;
               default:die($errorMsg);
           }
           //释放内存
           if(isset($water_info)) unset($water_info);
           if(isset($water_im)) imagedestroy($water_im);
           unset($ground_info);
           imagedestroy($ground_im);
       }