It'll be a problem because of PHP structure. But maybe use/create a keyword to receive the case value.

    switch ($a instanceof case) {
        case Foo : echo "Foo"; break;
        case Bar : echo "Bar"; break;
        default : echo "Unknown" : break;
    }

I'm going to mark this wontfix, on the original proposal, for three reasons:
1. It's a very specific use case
2. I imagine it would require a specific syntax "exception" in the PHP parser to support a switch+interfaceof+case class names
3. You can already do this:

switch (true) {
  case $a instanceof Foo: echo "Foo"; break;
  case $a instanceof Bar: echo "Bar"; break;
  default: echo "Unknown"; break;
}

Regarding using "case" in the condition, I think that's more feasible. @david.proweb, if you'd like to push for this feature then I suggest bringing it up on the internals mailing list for discussion.