For stream_wrappers, by calling fopen() with a context parameter and then fstat() on the opened stream, you should be able to pass a context (the ->context attribute in the wrapper's instance).

And it seems that StreamWrapper stat doesn't use default context either.

$options = array('testfs'=> array(
   'storagePath'=>dirname(ROOTPATH).DIRECTORY_SEPARATOR.'storage'
   )
);

$defaultContext = stream_context_get_options(stream_context_get_default());

$newContext = array_replace_recursive($defaultContext, $options);

stream_context_set_default($newContext);

stream_wrapper_register('testfs', 'Test_StreamWrapper');

class Test_StreamWrapper
{
  public $context;

  public function url_stat()
  {
    if($this->context === null)
    {
       print 'No Context!';   
    }
    return false;
  }
}

file_exists('testfs://foobar');

Expected:

Got:
No Context!

> but the "stat" family of functions is limited by the lack of a
> "context" param.

You should be able to use stream_context_set_default()[1].

> And it seems that StreamWrapper stat doesn't use default context
> either.

The behavior is documented[2].

[1] <https://www.php.net/stream_context_set_default>
[2] <https://www.php.net/manual/en/class.streamwrapper.php#streamwrapper.props>