Not at this time as far as I know. InterKat was working on one but then they were acquired by Adobe. Just a week before the acquisition announcment was made InterKat officially dropped work on a PHP extension for Expression Web.
There will be native support in future versions but we don't know which future version. You might want to put what sort of PHP support you are looking for in the Feature Request thread http://forum.by-expression.com/forums/thread/506.aspx I have arranged to chat with some of the product team at MIX 07 about what people post in the thumbs up,thumbs down and feature request threads. I would like to see items in all three categories.
Cheryl D Wise MS MVP Expression Instructor: starttoweb.com