php如何高效找出所有下级

<?php
 
$data = [
	[
	'uid' => 1,
	'username' => '155',
	'parent_username' => '0'
	],
	[
	'uid' => 2,
	'username' => '186',
	'parent_username' => '155'
	],
	[
	'uid' => 3,
	'username' => '189',
	'parent_username' => '186'
	],
	[
	'uid' => 4,
	'username' => '188',
	'parent_username' => '155'
	],
	[
	'uid' => 5,
	'username' => '187',
	'parent_username' => '188'
	],
	[
	'uid' => 6,
	'username' => '1898',
	'parent_username' => '146'
	],
];
 
/**
 * 获取所有下级
 * [getTree description]
 * @param [type] $data [description]
 * @param string $parent_username [description]
 * @return [type] [description]
 */
//有层次，返回所有下级数据
function getTree($data, $parent_username = '0')
{
 $arr = [];
	foreach($data as $key => $val){
	if($val['parent_username'] == $parent_username){
	$val['children'] = getTree($data, $val['username']);
	$arr[] = $val;
	}
	}
	return $arr;
}
 
//无层次，仅返回id
function getTree($data, $parent_username = '0', $is_first_time = true)
{
 static $arr = [];
 if ($is_first_time) {
 $arr = [];
 }
 foreach ($data as $key => $val) {
 if ($val['parent_username'] == $parent_username) {
 $arr[] = $val['uid'];
 getTree($data, $val['username'], false);
 }
 }
 return $arr;
}
 
echo '<pre>';
print_r(getTree($data));
 
 
 
?>