Can't use method return value in write context

I would think the following piece of code should work, but it doesn't **(Edited: Now works in PHP 5.5+)**: if (!empty($r->getError())) Where `getError()` is simply: public function getError() { return $this->error; } Yet I end up with this error: > can't use method return value in write context What does this mean? Isn't this just a read?