I'm not sure PHP's really up to the task - lacking closures might prove to be a big issue. It would probably be reasonably simple to make an arc interpreter, or an interpreter for a simpler intermediate language, but performance would probably suffer.
Why not write an Arc interpreter in Perl -- or even an interpreter for a web-centric subset of Arc? Stick the interpreter in the cgi-bin directory and use that to run your Arc scripts. Perl/CGI is even more ubiquitous on shared hosting accounts than PHP, supports closures, et cetera.
In fact, I've considered creating a new closures-based object system in Perl 5 in the past (never got around to it), which sounds suspiciously like it might have some things in common with some characteristics of Arc.