How to test a class that uses a Silex object?

Question

PHP

Hello.
Faced the problem of writing unit tests for classes that use the Silex framework object. I can't figure out how to write a mock for it.
I have a class:

class ProjectHelper
{
    private $app;

    function __construct ( Application $app )
    {
        $this -> app = $app;
    }

    public function GetProjectList ()
    {
        ...
        $row = $this -> app['db'] -> fetchAll ( $query );
        ...
    }
}

I want to test the GetProjectList method, so I need to put a mock on app['db'] -> fetchAll(), but how exactly do I do it?
This one doesn't want to work:

$appMock = $this -> getMockBuilder ( 'Silex\Application' )
            -> disableOriginalConstructor ()
            -> getMock ();

        $appMock -> expects ( $this -> once () )
            -> method ( 'fetchAll' )
            -> will ( $this -> returnValue ( 1 ) );

PHPUnit throws an error:

Trying to configure method "fetchAll" which cannot be configured because it does not exist, has not been specified, is final, or is static