PHP, get relevant URL of the current page

 In PHP project development, we often need to get some of the relevant URL of the current page, below is some tip:
 test URL:
 http://localhost/jiansenlu/index.php?userid=2
/ / Get the domain name or host address  localhost
  echo $ _SERVER ['HTTP_HOST']. "<br>";

 / / Get the web address  /jiansenlu/index.php 
echo $ _SERVER ['PHP_SELF']. "<br>";
 
 / / Get URL parameters userid=2 
 echo $ _SERVER ["QUERY_STRING"]. "<br>";
 
 / / Get user agent 
 echo $ _SERVER ['HTTP_REFERER']. "<br>";
 
 Get the full url 
 http://localhost/jiansenlu/index.php?userid=2
 echo 'http://'. $ _SERVER ['HTTP_HOST']. $ _SERVER ['REQUEST_URI'];
or  
 echo 'http://'. $ _SERVER ['HTTP_HOST']. $ _SERVER ['PHP_SELF']. '?'. $ _SERVER ['QUERY_STRING'];
 
 / / contains the port number of the complete url
http://localhost:80/jiansenlu/index.php?userid=2  
 echo 'http://'. $ _SERVER ['SERVER_NAME']. ':'. $ _SERVER ["SERVER_PORT"]. $ _SERVER ["REQUEST_URI"];

 / / only take the path  http://localhost/jiansenlu
$ url = 'http://'. $ _SERVER ['SERVER_NAME']. $ _SERVER ["REQUEST_URI"]; 
echo dirname ($ url);