PHP迭代与递归实现无限级分类

$arr = [
  1=>['id'=>1,'name'=>'父1','father'=>NULL],
  2=>['id'=>2,'name'=>'父2','father'=>NULL],
  3=>['id'=>3,'name'=>'父3','father'=>NULL],
  4=>['id'=>4,'name'=>'儿1-1','father'=>1],
  5=>['id'=>5,'name'=>'儿1-2','father'=>1],
  6=>['id'=>6,'name'=>'儿1-3','father'=>1],
  7=>['id'=>7,'name'=>'儿2-1','father'=>2],
  8=>['id'=>8,'name'=>'儿2-1','father'=>2],
  9=>['id'=>9,'name'=>'儿3-1','father'=>3],
  10=>['id'=>10,'name'=>'儿3-1-1','father'=>9],
  11=>['id'=>11,'name'=>'儿1-1-1','father'=>4],
  12=>['id'=>12,'name'=>'儿2-1-1','father'=>7],
];
function generateTree($items){
  $tree = array();
  foreach($items as $item){
    if(isset($items[$item['father']])){
      $items[$item['father']]['son'][] = &$items[$item['id']]; 
    }else{
      $tree[] = &$items[$item['id']];
    }
  }
  return $tree;
}
$tree = generateTree($arr);
print_r(json_encode($tree));

$arr = [
  0=>['id'=>1,'name'=>'父1','father'=>0],
  1=>['id'=>2,'name'=>'父2','father'=>0],
  2=>['id'=>3,'name'=>'父3','father'=>0],
  3=>['id'=>4,'name'=>'儿1-1','father'=>1],
  4=>['id'=>5,'name'=>'儿1-2','father'=>1],
  5=>['id'=>6,'name'=>'儿1-3','father'=>1],
  6=>['id'=>7,'name'=>'儿2-1','father'=>2],
  7=>['id'=>8,'name'=>'儿2-1','father'=>2],
  8=>['id'=>9,'name'=>'儿3-1','father'=>3],
  9=>['id'=>10,'name'=>'儿3-1-1','father'=>9],
  10=>['id'=>11,'name'=>'儿1-1-1','father'=>4],
  11=>['id'=>12,'name'=>'儿2-1-1','father'=>7],
];
function generateTree($arr,$id,$step){
  static $tree=[];
  foreach($arr as $key=>$val) {
    if($val['father'] == $id) {
      $flg = str_repeat('└―',$step);
      $val['name'] = $flg.$val['name'];
      $tree[] = $val;
      generateTree($arr , $val['id'] ,$step+1);
    }
  }
  return $tree;
}
$tree = generateTree($arr,0,0);
foreach ($tree as $val){
  echo $val['name'].'<br>';
}