Question

 $elements = $xpath->query(
   "//message[(@sender ='".$from." and @receiver = '".$username."') or
   (@receiver='unread' and @sender = '".$username."')]"
  );

Whats the problem with this code?
I need something like this SELECT data WHERE ((sender == from AND receiver = username)OR(sender == username AND receiver = from)) from my xml file

Answer 1

Use sprintf instead. It’s less messy and errors are more easy to spot.

$query = sprintf(
    '//message[
        (@sender = "%1$s" and @receiver = "%2$s") or
        (@receiver="unread" and @sender = "%2$s")
    ]',
    $from,
    $receiver
);

Also, make sure to sanitize $from and $receiver to prevent XPath Injection attacks.