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<?php $x = array('z'=>1,'a'=>1,'x'=>1,'b'=>1,'y'=>1,'c'=>1); var_dump(array_keys($x));
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(6) { [0]=> string(1) "z" [1]=> string(1) "a" [2]=> string(1) "x" [3]=> string(1) "b" [4]=> string(1) "y" [5]=> string(1) "c" }