XHTML Validator Problem

I've finally got round to making some attempt to validating the pages of a site I've done with PHP using the W3C XHTML Validator. However, it's throwing out errors wherever I use session control on pages. The server generates PHP Session ID variables which are added to links to other pages but my understanding is that these are rejected on the basis that the '&' and '=' characters are invalid (I might be wrong here). Examples of the error messages generated by the validator are given below: Line 154, column 102: cannot generate system identifier for general entity "PHPSESSID" ...u' href='competitions.php?option=add&PHPSESSID=888a059dbc17855c09885c3ea978df ^ Line 154, column 102: general entity "PHPSESSID" not defined and no default entity (explain... <http://validator.w3.org/docs/errors.html>). ...u' href='competitions.php?option=add&PHPSESSID=888a059dbc17855c09885c3ea978df ^ These are the only error messages left - having dealt with the dozens of others left in through my own sloppy coding :-( I'm not sure how to tackle this given that these elements of the page are generated automatically. Is there a way round this or are there any sources of information relating to this area? Thanks in advance, Michael Egan