PHPの再帰と無限分類の例の詳細な説明

";
$area = array(
array('id'=>1,'area'=>'北京','pid'=>0),
array('id'=>2,'area'=>'广西','pid'=>0),
array('id'=>3,'area'=>'广东','pid'=>0),
array('id'=>4,'area'=>'福建','pid'=>0),
array('id'=>11,'area'=>'朝阳区','pid'=>1),
array('id'=>12,'area'=>'海淀区','pid'=>1),
array('id'=>21,'area'=>'南宁市','pid'=>2),
array('id'=>45,'area'=>'福州市','pid'=>4),
array('id'=>113,'area'=>'亚运村','pid'=>11),
array('id'=>115,'area'=>'奥运村','pid'=>11),
array('id'=>234,'area'=>'武鸣县','pid'=>21)
); 

function t($arr,$pid=0,$lev=0){
 static $list = array();
 foreach($arr as $v){
 if($v['pid']==$pid){
  echo str_repeat("  ",$lev).$v['area']."
";
  //这里输出，是为了看效果
  $list[] = $v;
  t($arr,$v['id'],$lev+1);
 } 
 }
 return $list;
}
$list = t($area);
echo "
";
print_r($list);
?>