Facebook engineer Marcel Laverdet has written up the XHP project on which he has been working at Facebook, and which he says is “quickly becoming a cornerstone of front-end PHP development at Facebook”. XHP enables XML fragments to be valid PHP expressions. In addition, most HTML elements have been pre-defined as variables. The project is hosted on github and you can read a quick summary here. PHP inventor Rasmus Lerdorf has had a quick look and says:

The main interest, at least to me, is that because PHP now understands the XML it is outputting, filtering can be done in a context-sensitive manner. The input filtering built into PHP can not know which context a string is going to be used in. If you use a string inside an on-handler or a style attribute, for example, you need radically different filtering from it being used as regular XML PCDATA in the html body. Some will say this form is more readable as well, but that isn’t something that concerns me very much.

Lerdorf goes on to express concern about performance, but says that in combination with APC caching it is much better, and with HipHop compilation to native code becomes “a viable approach”.

One of the benefits here is that getting XML markup out of quoted strings means that it can be checked as it is parsed, so that errors are caught earlier. There are parallels with Microsoft’s work on Linq (Language Integrated Query) which pulls database queries into the language, and XML literals in Visual Basic 9.0. Ideally, developers should not have to hide code such as SQL, XML or Javascript within strings that are not checked for valid syntax until runtime.

This might also re-ignite the debate about whether XML literals should be in C#, now also gaining more attention because of the popularity of ASP.NET MVC.