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<?php var_dump(array_merge(['foo' => 'foo', 'bar' => null], ['foo' => null, 'bar' => 'bar']));
based on DiELH
Output for 5.6.0 - 5.6.30, hhvm-3.18.5 - 3.22.0, 7.1.0 - 7.3.0alpha1
array(2) { ["foo"]=> NULL ["bar"]=> string(3) "bar" }