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<?php $a = array('a' => 'one', 'b' => 'two'); var_dump(array_keys($a)); var_dump(array_keys($a, null));
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(2) { [0]=> string(1) "a" [1]=> string(1) "b" } array(0) { }