Get entire URL, including query string and anchor

// find out the domain:
$domain = $_SERVER['HTTP_HOST'];
// find out the path to the current file:
$path = $_SERVER['SCRIPT_NAME'];
// find out the QueryString:
$queryString = $_SERVER['QUERY_STRING'];
// put it all together:
$url = "http://" . $domain . $path . "?" . $queryString;
echo $url;

// An alternative way is to use REQUEST_URI instead of both
// SCRIPT_NAME and QUERY_STRING, if you don't need them seperate:
$url = "http://" . $domain . $_SERVER['REQUEST_URI'];
echo $url;

You can pass up the full URL, including the anchor (the part after the #), using a Javascript onload function that sends that URL to an Ajax endpoint.

You can't - you'll have to write the value of the hash to a cookie via Javascript to send it to the server on the subsequent request.

No, I am afraid not, since the hash (the string including the #) never gets passed to the server, it is solely a behavioural property of the browser. The $_SERVER['REQUEST_URI'] variable will contain the rest however.

If you really need to know what the hash is, you will have to use the document.location.hash JavaScript property, which contains the contents of the hash (you could then insert it in a form, or send it to the server with an ajax request).