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<?php $a = array('foo' => null, null => "abc"); var_dump(isset($a[null])); var_dump(isset($a['foo'])); var_dump((isset($a['foo']) || $a['foo'] === null));
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
bool(true) bool(false) bool(true)