Description:
------------
It seems that ArrayObject should implement ArrayAccess given that it already implements Traversable.  For me to know if a given variable can be accessed with the [] operator, I need to do:

if (is_array($obj) || $obj instanceof ArrayAccess || $obj instanceof ArrayObject) { }

Reproduce code:
---------------
function array_check($obj) {
	if (is_array($obj)) {
		print "Is an array()<br />\n";
	}
	if ($obj instanceof ArrayAccess) {
		print "Implements ArrayAccess<br />\n";
	}
	if ($obj instanceof ArrayObject) {
		print "Is an instance of ArrayObject<br />\n";
	}
}

array_check(new ArrayObject(array()));

Expected result:
----------------
Implements ArrayAccess
Is an instance of ArrayObject

Actual result:
--------------
Is an instance of ArrayObject