PHP合成透明图片

  //线下证书背面合成
    public function CreateOfflineCourseCertBack($type=1)
    {
          //盖章图片
         $mix_img=ROOT_PATH . 'public/uploads/certsimg/offline_cert_back_mix.png';
         
         //背景图片
         //使用课程
         $back_1 = ROOT_PATH . 'public/uploads/certsimg/offline_cert_back_1.png';
         //
         $back_2 = ROOT_PATH . 'public/uploads/certsimg/offline_cert_back_2.png';
 
         $type==1 && $path_1=$back_1;
         $type==2 && $path_1=$back_2;
 
         //创建图片对象
        $image_1 = imagecreatefrompng($path_1);
        $image_2 = imagecreatefrompng($mix_img);
        
        //合成图片
        imagecopy($image_1, $image_2, 471, 210, 0, 0, imagesx($image_2), imagesy($image_2));
        
        //这是要插入到图片的文字
        $orgcenter = '123培训中心';
        $orgaddress = '123科范德萨';
        $teacher = '李天一 202006061234';
 
        
        //文件名
        $filename="temp";
        
        //设置文字颜色
        $black = imagecolorallocate($image_1, 0, 0, 0);
        
        //写的文字用到的字体
        $font = ROOT_PATH . 'public/uploads/certsimg/simhei.ttf';
        
        //在图片里插入文字$black
        imagettftext($image_1, 20, 0, 161, 143, $black,$font, $orgcenter);
        imagettftext($image_1, 20, 0, 161, 193, $black,$font, $orgaddress);
        imagettftext($image_1, 20, 0, 161, 241, $black, $font, $teacher);
 
        
        //这是合成后的图片保存的路径
        $upload_dir = "public/uploads/certsimg/"; 
        
        imagepng($image_1, ROOT_PATH . $upload_dir . $filename . '.png');
     
        $finalurl='uploads/certsimg/'.$filename . '.png';
        
        return  $finalurl;
          
    }
}

效果如图

关键是那个章 要透明