offsetSet not being called in ArrayObject
**Example php code:**

    class TestArrayObject extends ArrayObject {
    	function offsetSet($index,$val){
    		echo $index.':'.$val.PHP_EOL;
    	}
    }
    
    $s = new TestArrayObject();
    
    //test 1
    $s['a'] = 'value';//calls offsetSet
    //test 2
    $s['b']['c'] = 'value';//does not call offsetSet, why?
    
    var_dump($s);

why doesn't test 2 call the offsetSet method?
offsetSet not being called in ArrayObject
===

**Example php code:**

    class TestArrayObject extends ArrayObject {
    	function offsetSet($index,$val){
    		echo $index.':'.$val.PHP_EOL;
    	}
    }
    
    $s = new TestArrayObject();
    
    //test 1
    $s['a'] = 'value';//calls offsetSet
    //test 2
    $s['b']['c'] = 'value';//does not call offsetSet, why?
    
    var_dump($s);

why doesn't test 2 call the offsetSet method?