Field not found warning

I have this simple class:

class TestClass {
    public $myvar;

    function __construct() {
        print_r($this->myvar);
    }
}

I am puzzles because PHPStorm highlights "myvar" as a warning with this comment:

"Field 'myvar' not found in class test"

I have tried File > Invalidate Cache to no avail.  

This is a Wordpress plugin project.  However the exact same file works fine (no warning) when included in an empty PHP project, so I think something must be wrong in my project settings.  Question is, what might it be??