+ call will return false, indicating no more valid tokens.
+
+ is_url_encoded is an out parameter. If not NULL, a boolean value will be
+ stored into it, letting the caller know whether or not the extracted value is
+ URL-encoded. The caller can then decode it with url_unescape(), which however
+ performs decoding in-place. URL-encoding is used by RFC 2231 to support
+ non-US-ASCII characters in HTTP header values. */