PHP无限级分类实现（递归+非递归）

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<?php  /**  * Created by PhpStorm.  * User: qishou  * Date: 15-8-2  * Time: 上午12:00  */  //准备数组，代替从数据库中检索出的数据(共有三个必须字段id,name,pid)  header("content-type:text/html;charset=utf-8");  $categories = array(      array('id'=>1,'name'=>'电脑','pid'=>0),      array('id'=>2,'name'=>'手机','pid'=>0),      array('id'=>3,'name'=>'笔记本','pid'=>1),      array('id'=>4,'name'=>'台式机','pid'=>1),      array('id'=>5,'name'=>'智能机','pid'=>2),      array('id'=>6,'name'=>'功能机','pid'=>2),      array('id'=>7,'name'=>'超级本','pid'=>3),      array('id'=>8,'name'=>'游戏本','pid'=>3),  );      /*======================非递归实现========================*/  $tree = array();  //第一步，将分类id作为数组key,并创建children单元  foreach($categories as $category){      $tree[$category['id']] = $category;      $tree[$category['id']]['children'] = array();  }  //第二步，利用引用，将每个分类添加到父类children数组中，这样一次遍历即可形成树形结构。  foreach($tree as $key=>$item){      if($item['pid'] != 0){          $tree[$item['pid']]['children'][] = &$tree[$key];//注意：此处必须传引用否则结果不对          if($tree[$key]['children'] == null){              unset($tree[$key]['children']); //如果children为空，则删除该children元素（可选）          }      }  }  ////第三步，删除无用的非根节点数据  foreach($tree as $key=>$category){      if($category['pid'] != 0){          unset($tree[$key]);      }  }      print_r($tree);      /*======================递归实现========================*/  $tree = $categories;  function get_attr($a,$pid){      $tree = array();                                //每次都声明一个新数组用来放子元素      foreach($a as $v){          if($v['pid'] == $pid){                      //匹配子记录              $v['children'] = get_attr($a,$v['id']); //递归获取子记录              if($v['children'] == null){                  unset($v['children']);             //如果子元素为空则unset()进行删除，说明已经到该分支的最后一个元素了（可选）              }              $tree[] = $v;                           //将记录存入新数组          }      }      return $tree;                                  //返回新数组  }  echo "<br/><br/><br/>";      print_r(get_attr($tree,0));