In a view I tried to render an Array which of course gets wrapped into an sfOutputEscaperArrayDecorator. So the following would not work, since the array became an object:
if (array_key_exists($cell->getCoordinate(), $bildArr)) {

In order to achieve the same thing one can use offsetExists() which can be called on the object instance:
if ($bildArr->offsetExists($cell->getCoordinate())) {

Apart from that the object can be used like a normal array.