<?php
 
$items = [
  'item_a' => ['require' => ['item_b']],
  'item_b' => ['require' => []],
  'item_c' => ['require' => ['item_a']],
  'item_d' => ['require' => ['item_a', 'item_e']],
  'item_e' => ['require' => ['item_a']],
];
 
foreach (array_keys($items) as $key) {
	$items[$key]['name'] = $key;
}
 
uasort($items, function($a, $b) {
	return in_array($b['name'], $a['require']) || empty($b['require']);
}); 
 
foreach (array_keys($items) as $key) {
	unset($items[$key]['name']);
	echo $key . PHP_EOL;
}

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