Description:
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Array_key_exists accepts ArrayObject since PHP 5.0.0, but 5.3.0 refuse it. Feature or bug?

Reproduce code:
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$arr = array('a' => 10);

$obj = new ArrayObject($arr);

echo array_key_exists('a', $obj);


Expected result:
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TRUE

Actual result:
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Warning: array_key_exists() expects parameter 2 to be array, object given
NULL