It looks like a PHP extension to embed JavaScript using the V8 engine. The docs are pretty limited. The return value of executing a blob of JS is described thusly: "Returns the last variable instantiated in the Javascript code converted to matching PHP variable type." I supposed the mapping is obvious, to someone :). I'd personally want to know if JS objects come back as stdClass objects or assoc arrays.

It could definitely be handy for a lot of reasons. Seems like it might be handy for embedding a bit of JavaScript testing in your PHP test suite.