How to implement hierarchical tree expansion recursively in PHP

<?php 
  
$db = mysql_connect(&#39;localhost&#39;, &#39;root&#39;, &#39;root&#39;) or die(&#39;Can\&#39;t connect to database&#39;); 
mysql_select_db(&#39;test&#39;) or die(&#39;Can\&#39;t find database : test&#39;); 
$result = mysql_query(&#39;select id, fid, name from tree&#39;); 
while($arr = mysql_fetch_array($result)){ 
  $data[] = array( 
    &#39;id&#39; => $arr[&#39;id&#39;],  
    &#39;fid&#39; => $arr[&#39;fid&#39;], 
    &#39;name&#39; => $arr[&#39;name&#39;],  
  ); 
} 
  
// 将数据按照缩进简单排列 见图1 
function data2arr($tree, $rootId = 0, $level = 0) { 
  foreach($tree as $leaf) { 
    if($leaf[&#39;fid&#39;] == $rootId) { 
      echo str_repeat(&#39;    &#39;, $level) . $leaf[&#39;id&#39;] . &#39; &#39; . $leaf[&#39;name&#39;] . &#39;<br/>&#39;; 
      foreach($tree as $l) { 
        if($l[&#39;fid&#39;] == $leaf[&#39;id&#39;]) { 
          data2arr($tree, $leaf[&#39;id&#39;], $level + 1); 
          break; 
        } 
      } 
    } 
  } 
} 
  
data2arr($data); 
echo &#39;<br/>-----------------------------------------------------------------------<br/>&#39;; 
  
// 将数据按照所属关系封装 见图2 
function arr2tree($tree, $rootId = 0) { 
  $return = array(); 
  foreach($tree as $leaf) { 
    if($leaf[&#39;fid&#39;] == $rootId) { 
      foreach($tree as $subleaf) { 
        if($subleaf[&#39;fid&#39;] == $leaf[&#39;id&#39;]) { 
          $leaf[&#39;children&#39;] = arr2tree($tree, $leaf[&#39;id&#39;]); 
          break; 
        } 
      } 
      $return[] = $leaf; 
    } 
  } 
  return $return; 
} 
  
$tree = arr2tree($data); 
print_r($tree); 
echo &#39;<br/>-----------------------------------------------------------------------<br/>&#39;; 
  
// 将数据使用HTML再次展现 见图3 
function tree2html($tree) { 
  echo &#39;<ul>&#39;; 
  foreach($tree as $leaf) { 
    echo &#39;<li>&#39; .$leaf[&#39;name&#39;]; 
    if(! emptyempty($leaf[&#39;children&#39;])) tree2html($leaf[&#39;children&#39;]); 
    echo &#39;</li>&#39;; 
  } 
  echo &#39;</ul>&#39;; 
} 
  
tree2html($tree);