A session is generated with session_start(). The browser is set not to accept cookies. Inside the HTML-Code without the Session there would appear a code like:

<a href="/index.php?entryid=0&amp;category=0"></a>

With the automatic adding of the session-id it will look like:

<a href="/index.php?entryid=0&amp;category=0&PHPSESSID=6865f7d1491ccaddd676430ede3328"></a>

According to http://validator.w3.org/check  this does not qualify as valid HTML:

" The validator has found an entity (something like &this;) that it doesn't recognize. There are a few possibilities:

      A reference to a URI that uses & as a separator between parameters, such as "http://example.org/prog?x=1&y=2".

      To solve this problem, simply replace all the &'s in attribute values with &amp; (user agents will convert them back before following the links.) "