As of 5.3 (and I won't say prior to that because even I don't believe it), PHP has become a nice utility language for just more than serving out wordpress.
Closures and the reference sweep GC level the playing field.
Building a Lisp on top of PHP strikes me as a vehicle for helping developers to adhere to the "good parts." I'm not sure whether that was the intent here, but it could turn out to be a useful side effect.
A note on the language itself:
It appears that you can choose any set of matching braces that you want anywhere in your program. I think this is the convention Scheme uses. Common Lisp reserves braces other than () for reader macros.
I personally, however, favor Clojure's approach of defining distinct behavior for each brace type. There are only three pairs of matched characters on the keyboard. It seems to me that this makes them too valuable to not leverage them as a core part of the language. Immediately knowing the meaning of a specific brace type makes it much easier to read someone else's code.
I have seen quiet a few PHP bashing comments recently that while true of older versions are much less true of 5.3, I mean there is still the same old nitpicks about the parameter orders and the amount of built in functions in the standard namespace but it is defiantly heading in the right direction.
An embedded lisp, even a slow and imperfect one is a nice stepping stone to have though.
It takes away two thirds of the trouble you have to go through when starting with a new language, environment and libraries.