Using version IU-140.2683, I get this error: Following plugins are incompatible with current IDE build: PHP

Please switch to the release version or wait until the compatible plug-in is released.

this begs the question - why release an EAP when the major plugins are NOT available yet?  i think it is a mistake.

That is precisely why this is an EAP.
EAP means that it is not considered production ready, and may contain bugs that cause data-loss or other major problems.

I think the consistently high quality of the JetBrains EAP releases has caused some people to lose sight of the fact that this is an Early Access program, and is intended primarily as a way to get feedback from the public on features or problems.
It is also intended for plugin authors to have a chance to update their plugins to the new version before it becomes an official release.
This includes non-bundled plugins that happen to be developed by other JetBrains teams. They have their own schedule aside from the main IDEA build stream, and may lag a bit (or a lot) behind the latest EAP release, just like any third-party plugin.

If you need a stable production environment to work in (I do), try to hold off on installing EAP releases, or install the EAP seperately so that your production environment remains stable.

I understand that it's tempting to try the latest productivity boosts that are being built, but please be aware of the nature of an Early Access release: it may break stuff.

it all depends on what kind of feedback you're trying to get.  is the EAP a beta program (where one would have a reasonable expectation of major plugins being available)?  or an alpha program?  your comment that the EAP should be used by plugin devs to adapt to a new release strongly suggests an alpha release, not a beta.  and that does set a very different expectation.  thanks for the clarification.

/mark