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?

Can’t use method return value in write context