<?php

$array = [
	'd' => [
		'f',
		'e',
	],
	'a' => [
		'c',
		'b',
	],
];

function recur_asort(&$array) {
   foreach ($array as &$value) {
      if (is_array($value)) recur_asort($value);
   }
   return asort($array);
}

recur_asort($array);

var_dump($array);