You could fix your function, but symfony already has this functioniality:
function get_base_url() {
$request = sfContext::getInstance()->getRequest();
return $request->getRelativeUrlRoot();
}
This returns: string ‘/project/web’ (length=12)

If you want the absolute URL with http://servername/…, use this:
function get_base_url() {
$request = sfContext::getInstance()->getRequest();
return $request->getUriPrefix() . $request->getRelativeUrlRoot();
}
string ‘http://localhost/project/web’ (length=28)