The arg-separator.output default value of "&" causes an invalid HTML/XHTML page to be displayed when PHP attaches the PHPSESSID to any hrefs on the page that already have a querystring.

eg:

<a href="example.php?page=1">

becomes:

<a href="example.php?page=1&PHPSESSID=xxxxxxxxxxx">

which is invalid HTML.  It should be:

<a href="example.php?page=1&amp;PHPSESSID=xxxxxxxxxxx">

This issue causes problems when sending pages as application/xhtml+xml as Mozilla will detect the error and refuse to display the page.

Changing the default value (to "&amp;") should theoretically not cause any backwards compatibility issues since it gets translated (to "&") by the web browser.  It will however, help those of us who _do not_ have access to the PHP configuration files on the web server.