php如何实现图片合成并生成水印（完整代码）

<?php
$dst_path ='./upload/goods/2018/06-04/2bd2518a2e4e6d8bdab9f01434b9f754.png';
$src_path = './public/upload/weixin/20180620/a643cf1ee7dd5bda12270647ca543f60.png';
$hz = substr(strrchr($dst_path, '.'), 1);
$image = date("YmdHis").rand(100000,999999).".".$hz;
//创建图片的实例
$dst = imagecreatefromstring(file_get_contents($dst_path));
$src = imagecreatefromstring(file_get_contents($src_path));
//获取水印图片的宽高
$src_w =139;$src_h=58;
list($src_w,$src_h) = getimagesize($src_path);
//如果水印图片本身带透明色，则使用imagecopy方法
imagecopy($dst, $src, 85,85, 0, 0, $src_w, $src_h);
//输出图片
list($src_w, $src_h, $dst_type) = getimagesize($dst_path);
switch ($dst_type) {
    case 1://GIF

        header('Content-Type: image/gif');

        imagegif($dst,".".$image);        
        break;    
    case 2://JPG

        header('Content-Type: image/jpeg'); 

        imagejpeg($dst,".".$image); 

        break; 

    case 3://PNG 

        header('Content-Type: image/png'); 

        imagepng($dst,".".$image); 

        break; 

    default: 

        break;
} 

imagedestroy($dst); 

imagedestroy($src); 

return $image;