将父URL复制到php变量

The following code copy the URL of my "index.php" page

$url = "http://".$_SERVER['SERVER_NAME']."/".$_SERVER['REQUEST_URI'];

But I need to add this code to another website in iframe, so I need to get the URL of the other website as a parent URL

I guess you're talking about the referring URL. Use this

$_SERVER['HTTP_REFERER']

in the iframed page