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<?php $input = [ 'firstName' => null, 'lastName' => null, 'category' => [ 'name' => null, 'service' => [ 'foo' => [ 'bar' => null ] ] ] ]; function prepareFields(array $array): array { return array_reduce(array_keys($array), function ($result, $key) use ($array) { $result[] = $key; if (is_array($array[$key])) { $result[$key] = prepareFields($array[$key]); } return $result; }); } print_r(prepareFields($input));
Output for git.master, git.master_jit, rfc.property-hooks
Array ( [0] => firstName [1] => lastName [2] => category [category] => Array ( [0] => name [1] => service [service] => Array ( [0] => foo [foo] => Array ( [0] => bar ) ) ) )

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