Description:
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PHP 5.4 brings the new JsonSerializable interface. ArrayObject already works with json_encode as expected but does not explicitly implement this interface.

Test script:
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$ao = new ArrayObject(array('foo' => 'bar'));

var_dump(json_encode($ao));
var_dump($ao instanceof JsonSerializable);
var_dump($ao->jsonSerialize());

Expected result:
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string(13) "{"foo":"bar"}"
bool(true)
string(13) "{"foo":"bar"}"

Actual result:
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string(13) "{"foo":"bar"}"
bool(false)
PHP Fatal error:  Call to undefined method ArrayObject::jsonSerialize() in - on line 6