Tree traversal

<?php

$dfs = function ($tree) use (&$dfs) {
  $name = $tree[0];
  $children = $tree[1] ?? null;
  echo "$name \n";
  if (!$children) {
    return;
  }
  return array_map($dfs, $children);
};

$tree = ['A', [
  ['B', [['E'], ['F']]],
  ['C'],
  ['D', [['G'], ['J']]],
]];

$dfs($tree);
// => A
// => B
// => E
// => F
// => C
// => D
// => G
// => J