I think the majority of PHP developers could handle Erlang. The core language is small, the syntax maybe at times quirky but it is quite clear, the documentation is good and the community is welcoming. There is perhaps some extra effort in starting a project but with rebar I don't think it's any more onerous than the hoop-jumping involved in using a PHP framework.

If there are any PHP developers reading this who have tried and failed to pickup Erlang I'd be interested to know what got in your way.