PHP图片加水印实现方法，

<&#63;php
echo img_water_mark("2008112023204423477802.gif", "copyImg.png", $savepath=null, $savename="123.gif", $positon=2, $alpha=60);
/**
 * 图片加水印(适用于png/jpg/gif格式)
 * 
 * @author flynetcn
 *
 * @param $srcImg 原图片
 * @param $waterImg 水印图片
 * @param $savepath 保存路径
 * @param $savename 保存名字
 * @param $positon 水印位置 
 * 1:顶部居左, 2:顶部居右, 3:居中, 4:底部局左, 5:底部居右 
 * @param $alpha 透明度 -- 0:完全透明, 100:完全不透明
 * 
 * @return 成功 -- 加水印后的新图片地址
 *     失败 -- -1:原文件不存在, -2:水印图片不存在, -3:原文件图像对象建立失败-4:水印文件图像对象建立失败, -5:加水印后的新图片保存失败
 */
function img_water_mark($srcImg, $waterImg, $savepath=null, $savename=null, $positon=5, $alpha=30)
{
  $temp = pathinfo($srcImg);
  $name = $temp['basename'];
  $path = $temp['dirname'];
  $exte = $temp['extension'];
  $savename = $savename &#63; $savename : $name;
  $savepath = $savepath &#63; $savepath : $path;
  $savefile = $savepath .'/'. $savename;
  $srcinfo = @getimagesize($srcImg);
  if (!$srcinfo) {
    return -1; //原文件不存在
  }
  $waterinfo = @getimagesize($waterImg);
  if (!$waterinfo) {
    return -2; //水印图片不存在
  }
  $srcImgObj = image_create_from_ext($srcImg);
  if (!$srcImgObj) {
    return -3; //原文件图像对象建立失败
  }
  $waterImgObj = image_create_from_ext($waterImg);
  if (!$waterImgObj) {
    return -4; //水印文件图像对象建立失败
  }
  switch ($positon) {
  //1顶部居左
  case 1: $x=$y=0; break;
  //2顶部居右
  case 2: $x = $srcinfo[0]-$waterinfo[0]; $y = 0; break;
  //3居中
  case 3: $x = ($srcinfo[0]-$waterinfo[0])/2; $y = ($srcinfo[1]-$waterinfo[1])/2; break;
  //4底部居左
  case 4: $x = 0; $y = $srcinfo[1]-$waterinfo[1]; break;
  //5底部居右
  case 5: $x = $srcinfo[0]-$waterinfo[0]; $y = $srcinfo[1]-$waterinfo[1]; break;
  default: $x=$y=0;
  }
  imagecopymerge($srcImgObj, $waterImgObj, $x, $y, 0, 0, $waterinfo[0], $waterinfo[1], $alpha);
  switch ($srcinfo[2]) {
  case 1: imagegif($srcImgObj, $savefile); break;
  case 2: imagejpeg($srcImgObj, $savefile); break;
  case 3: imagepng($srcImgObj, $savefile); break;
  default: return -5; //保存失败
  }
  imagedestroy($srcImgObj);
  imagedestroy($waterImgObj);
  return $savefile;
}
function image_create_from_ext($imgfile)
{
  $info = getimagesize($imgfile);
  $im = null;
  switch ($info[2]) {
  case 1: $im=imagecreatefromgif($imgfile); break;
  case 2: $im=imagecreatefromjpeg($imgfile); break;
  case 3: $im=imagecreatefrompng($imgfile); break;
  }
  return $im;
}
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