Flycheck report PHP syntax errors

The combination of Polymode and Flycheck works by accident because PHP has a syntactical property that ignores the outside of ``.

However, the following cases are exceptions:

* The code contains a `declare` statement.
* Missing `?>` at the end of the code.

The solution of this problem requires the cooperation of both Flycheck and Polymode.

For example, it should be reasonably possible by reusing the buffer possessed by Polymode.