Php url help

If you mean controlling block visibility based on the url/path is pretty straight forward. If you don't have clean urls your paths are going to look like http://www.example.com/index.php?q=some/path/to/act/on, with clean urls that would be http://www.example.com/some/path/to/act/on. The part of the url after your sites domain is a Drupal path. The first part (some) can be retrieved with arg(0), the nect (path) with arg(1), etc. So for example to show a block that appears when showing a single node (any one) you could do something like

// Look for paths of the form node/{nid} where {nid} is a number
// Note we are going to return false if the path has any additional
// parts as in the case of editing the node.  You can
// drop the '&& ! arg(2)' part if you want it to return true when editing a node

if ( arg(0) == 'node' && is_numeric(arg(1)) && ! arg(2) ) {
  return TRUE;
}
else {
  return FALSE;
}

Note arg() returns the parts of the unaliased path.