One of the blockers for the effort to package HHVM for Debian is the licensing of the JSON extension. This is the same problem that has afflicted PHP (see <https://bugs.php.net/bug.php?id=63520>: "JSON extension includes a problematic license statement") and that we have discussed in the context of bug 47413 (<https://bugzilla.wikimedia.org/show_bug.cgi?id=47431>: "JSON extension dependency has a non-free component".
There is a drop-in replacement which is not encumbered with licensing issues: Remi Collet's pecl-json-c, available at <https://github.com/remicollet/pecl-json-c>. Paul Tarjan replied on the packaging thread (see <https://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg1208263.html>), saying that the HHVM team would be interested in swapping out the JSON library and replacing it with pecl-json-c, if someone were to port it. Since both extensions provide the same API, I imagine "porting" in this context means tweaking the include paths and the build configuration.