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<?php $foo = [ 1 => 'foo', 3 => 'bar', ]; var_dump(array_merge([null => ''], $foo));
based on OWlp2
Output for 5.6.0 - 5.6.30, hhvm-3.15.4, 7.0.0 - 7.2.0
array(3) { [""]=> string(0) "" [0]=> string(3) "foo" [1]=> string(3) "bar" }