Dynamically Typed has a new posting today discussing one of the less-known PHP configuration directives, arg_separator.output.

PHP's configuration directive arg_separator.output allows you to tell PHP how it should separate arguments in a URL and has a default value of '&'.

The directive affects all URLs that are generated or modified automatically by PHP. The only time this is likely to affect us is when we use PHP Session Handling along with session.use-trans-sid to auto-generate URLs with session IDs. So, if you don't use this, the following problem may not affect you.

They note that by changing this setting, you can (unfortunately) alter the way that PHP accepts information on the URL line which can lead to troubles down the road with XHTML/HTML validation of your documents (PHP gets confused between & and just a &)...