Convert the returned data set into an array tree

like
$old = array(
array('id'=>1,'pid'=>0,'name'=>'first' ),
array('id'=>2,'pid'=>1,'name'=>'second' ) ,
array('id'=>3,'pid'=>2,'name'=>'third'),
);
print_r(list_to_tree($old,'id','pid','_child'));
The output is as follows
$old = array(
array(
'id'=>1,
'pid'=>0,
'name'=>'first',
'_child'=>array(
'id'=>2,
' pid'=>1,
'name'=>'second'
'_child'=>array('id'=>3,'pid'=>2,'name'=>'third' ),
),
) ,

);

/**
 * Convert the returned data set into a Tree
 * @access public
 * @param array $list The data set to be converted
 * @param string $pid parent tag field
 * @param string $level level tag field
 * @ return array
*/
function list_to_tree($list, $pk='id',$pid = 'pid',$child = '_child',$root=0) {
 // Create Tree
 $tree = array();
 if(is_array($list)) {
 // Create an array reference based on the primary key
 $refer = array();
 foreach ($list as $key => $data) {
 $refer[$data[$pk]] =& $list[$key];
 }
 foreach ($list as $key => $data) {
 // Determine whether parent exists
 $parentId = $ data[$pid];
 if ($root == $parentId) {
 $tree[] =& $list[$key];
 }else{
 if (isset($refer[$parentId])) {
 $parent =& $refer[$parentId];
 $parent[$child][] =& $list[$key];
 }
 }
 }
 }
 return $tree;
}