Aside from novelty, one real world use might be an extension to SimpleTest's web testing capabilities. With a little work there's also potential to create an awesome Javascript source compressor'"the lexing patterns are all there.
I kid you not. In fact this looks like a pretty serious attempt. You'll need to download and run yourself. There's grammar rules for Javascript in there and a Javascript runtime written in PHP. It passes the "it works" test'"the examples run straight out of $ unzip. Looking at what it does and the TODO list, aside from the Unicode issue strikes me J4P5 is already more than 50% of the way there.
He also mentions another project, metaphp, that builds a "higher level langauge" on top of PHP. The parser for it was used in the J4P5 project mentioned above...
