<?php $a = ["I", "need", "this", "to", "be", "nested"]; return $prevArray ? [$key => $prevArray] : [$key]; }, null);
Standard input is empty
Array ( [I] => Array ( [need] => Array ( [this] => Array ( [to] => Array ( [be] => Array ( [0] => nested ) ) ) ) ) ) array(1) { ["I"]=> array(1) { ["need"]=> array(1) { ["this"]=> array(1) { ["to"]=> array(1) { ["be"]=> array(1) { [0]=> string(6) "nested" } } } } } }