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<?php $a = null; $a['foo']['bar'] = 5; var_dump($a);
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(1) { ["foo"]=> array(1) { ["bar"]=> int(5) } }