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<?php $input_array = array("FirSt" => 1, "SecOnd" => 4); var_dump(array_change_key_case($input_array, CASE_UPPER));
based on n3EXk
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(2) { ["FIRST"]=> int(1) ["SECOND"]=> int(4) }