哪位提供一个php的加水印程序，最好有详细的注释，我不太懂程序

//参数分别是：源图,生成文件,生成位置,水印文件,水印文本,背景色

function watermark($source, $target = '', $w_pos = 0, $w_img = '', $w_text = '', $w_font = 12, $w_color = '#cccccc')
{
if(!$this->watermark_enable || !$this->check($source)) return false;
if(!$target) $target = $source;
if ($w_img == '' && $w_text == '')
$w_img = $this->w_img;
$source_info = getimagesize($source);
$source_w = $source_info[0]; //获取宽
$source_h = $source_info[1]; //获取高
if($source_w < $this->w_minwidth || $source_h < $this->w_minheight) return false; //宽和高达不到要求直接返回
switch($source_info[2]) //新建图片
{
case 1 :
$source_img = imagecreatefromgif($source);
break;
case 2 :
$source_img = imagecreatefromjpeg($source);
break;
case 3 :
$source_img = imagecreatefrompng($source);
break;
default :
r