To send data to a remote PHP script, you must send a sf::Http request, either in POST or GET mode. GET encodes the variables in the URL, while POST hides them in the request body. On the PHP side, the difference is just a matter of reading the $_POST or $_GET global variable.
Then your PHP script can answer anything, for example "ok" or "error".
sf::Http::Request request;
request.setMethod(sf::Http::Request::Post);
request.setURI("/login.php");
request.setBody("username=xxx&password=yyy"); // I let you figure out how to build this string dynamically
sf::Http Http("www.mysite.org");
sf::Http::Response response = Http.sendRequest(request);
bool ok = (response.getBody() == "ok");
The PHP script:
$username = $_POST['username'];
$password = $_POST['password'];
// sql stuff with $username and $password...
if (ok)
echo "ok";
else
echo "error";
It might not work out of the box, I just wrote this quickly to show you the right direction.