Giant rewrites don't have the best track record of success. Other than language aesthetics, what would be the justification for undertaking that enormous cost and risk? I despise PHP but I wouldn't make that call.
Why can't something like PHP to Java be done? It's been done for everything to Javascript.
That way they could simple move PHP to the other language, and write whatever new in the other language?
It's not like javascript where you dont control the client, you control everything that is running on your servers.
HHVM is good for Facebook,though i fail to understand how they could not migrate to any other serverside language given the number of developpers they hire, unless the legacy code is an unmaintainable,unreadble mess. Twitter did it(rails to jvm).