Prior to HTML Purifier 1.2.0, this library blithely accepted user input that looked like this:
<a id="fragment">Anchor</a>
...presenting an attractive vector for those that would destroy standards compliance: simply set the ID to one that is already used elsewhere in the document and voila: validation breaks. There was a half-hearted attempt to prevent this by allowing users to blacklist IDs, but I suspect that no one really bothered, and thus, with the release of 1.2.0, IDs are now removed by default.
IDs, however, are quite useful functionality to have, so if users start complaining about broken anchors you'll probably want to turn them back on with %HTML.EnableAttrID. But before you go mucking around with the config object, it's probably worth to take some precautions to keep your page validating. Why?
You have (currently) four ways of dealing with the problem.
Keeping in terms with the KISS principle, let us deal with the most obvious solution: preventing users from using any IDs that appear elsewhere on the document. The method is simple:
$config->set('HTML', 'EnableAttrID', true);
$config->set('Attr', 'IDBlacklist' array(
'list', 'of', 'attribute', 'values', 'that', 'are', 'forbidden'
));
That being said, there are some notable drawbacks. First of all, you have to know precisely which IDs are being used by the HTML surrounding the user code. This is easier said than done: quite often the page designer and the system coder work separately, so the designer has to constantly be talking with the coder whenever he decides to add a new anchor. Miss one and you open yourself to possible standards-compliance issues.
Furthermore, this position becomes untenable when a single web page must hold multiple portions of user-submitted content. Since there's obviously no way to find out before-hand what IDs users will use, the blacklist is helpless. And since HTML Purifier validates each segment separately, perhaps doing so at different times, it would be extremely difficult to dynamically update the blacklist in between runs.
Finally, simply destroying the ID is extremely un-userfriendly behavior: after all, they might have simply specified a duplicate ID by accident.
Thus, we get to our second method.
This method, too, is quite simple: add a prefix to all user IDs. With this code:
$config->set('HTML', 'EnableAttrID', true);
$config->set('Attr', 'IDPrefix', 'user_');
...this:
<a id="foobar">Anchor!</a>
...turns into:
<a id="user_foobar">Anchor!</a>
As long as you don't have any IDs that start with user_, collisions are guaranteed not to happen. The drawback is obvious: if a user submits id="foobar", they probably expect to be able to reference their page with #foobar. You'll have to tell them, "No, that doesn't work, you have to add user_ to the beginning."
And yes, things get hairier. Even with a nice prefix, we still have done nothing about multiple HTML Purifier outputs on one page. Thus, we have a second configuration value to piggy-back off of: %Attr.IDPrefixLocal:
$config->set('Attr', 'IDPrefixLocal', 'comment' . $id . '_');
This new attributes does nothing but append on to regular IDPrefix, but is special in that it is volatile: it's value is determined at run-time and cannot possibly be cordoned into, say, a .ini config file. As for what to put into the directive, is up to you, but I would recommend the ID number the text has been assigned in the database. Whatever you pick, however, it has to be unique and stable for the text you are validating. Note, however, that we require that %Attr.IDPrefix be set before you use this directive.
And also remember: the user has to know what this prefix is too!
You may not want to bother. That's okay too, just don't enable IDs.
Personally, I would take this road whenever user-submitted content would be possibly be shown together on one page. Why a blog comment would need to use anchors is beyond me.
To revert back to pre-1.2.0 behavior, simply:
$config->set('HTML', 'EnableAttrID', true);
Don't come crying to me when your page mysteriously stops validating, though.