<?php $arr = [ 'A' => ['A1' => '', 'A2' => ''], 'B' => ['B1' => ['B1a' => '']] ]; echo 'Result: ', $result;
Standard input is empty
Array ( [A] => Array ( [A1] => [A2] => ) [B] => Array ( [B1] => Array ( [B1a] => ) ) ) Result: A, A1, A2, B, B1, B1a