代码如下:

//取重定向的地址
 class RedirectUrl{
     //地址
     var $url;
     //初始化地址
     function RedirectUrl($url){
         $this->url = $url;
     }
     /**
      * get_redirect_url()
      * 取重定向的地址
      *
      * @param string $url
      * @return string
      */
     private function get_redirect_url($url){
         $redirect_url = null;

         $url_parts = @parse_url($url);
         if (!$url_parts) return false;
         if (!isset($url_parts['host'])) return false; //can't process relative URLs
         if (!isset($url_parts['path'])) $url_parts['path'] = '/';

         $sock = fsockopen($url_parts['host'], (isset($url_parts['port']) ? (int)$url_parts['port'] : 80), $errno, $errstr, 30);
         if (!$sock) return false;

         $request = "HEAD " . $url_parts['path'] . (isset($url_parts['query']) ?'?'.$url_parts['query'] : '') . " HTTP/1.1\r\n";
         $request .= 'Host: ' . $url_parts['host'] . "\r\n";
         $request .= "Connection: Close\r\n\r\n";
         fwrite($sock, $request);
         $response = '';
         while(!feof($sock)) $response .= fread($sock, 8192);
         fclose($sock);

         if (preg_match('/^Location: (.+?)$/m', $response, $matches)){
             return trim($matches[1]);
         } else {
             return false;
         }
     }

     /**
      * get_all_redirects()
      * 取所有重定向地址
      *
      * @param string $url
      * @return array
      */
     private function get_all_redirects($url){
         $redirects = array();
         while ($newurl = $this->get_redirect_url($url)){
             if (in_array($newurl, $redirects)){
                 break;
             }
             $redirects[] = $newurl;
             $url = $newurl;
         }
         return $redirects;
     }

     /**
      * get_final_url()
      * 取实际地址
      *
      * @param string $url
      * @return string
      */
     function get_final_url(){
         $redirects = $this->get_all_redirects($this->url);

         if (count($redirects)>0){
             return array_pop($redirects);
         } else {
             return $this->url;
         }
     }
 }

 /**
  * get_show_pic
  * 取最终要显示的图片地址
  *
  * @param string $url
  * @return  string
  */
 function get_show_pic($url,$noimg ='noimg.gif',$newimg="https://cache.yisu.com/upload/information/20201209/266/43231.jpg"){
     $obj = new RedirectUrl($url);

     $realurl = $obj->get_final_url();

     if(strpos($realurl,$noimg)>0){
         return $newimg;
     }
     return $url;
 }
 //参考
 echo "";
 ?>