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<?php $array = array('a' => 'a1', 'b'=>'b2', 'c'=> 'b3', 'd4'); var_dump(array_keys($array));
Output for 4.3.0 - 5.6.21, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(4) { [0]=> string(1) "a" [1]=> string(1) "b" [2]=> string(1) "c" [3]=> int(0) }