php静态变量什么时候销毁

/**
 * 获取所有下级 
 * $data 用户数组 
 * $pid 用户user_id
 * $level 递归层次 
 * $bool_level 判断是否分级(不分级则获取所有的下级)
 * $clear 是否销毁静态变量
 */
 public function getTree($data, $pid = 0, $level = 0, $bool_level = true ,$clear=false ) {
 
 if ($pid < 1) {
 return false;
 }
 
 static $arr = [];
 
 if($clear){
 $arr = null;
 static $arr = [];
 }
 
 if ($bool_level) {
 $level = $level - 1;
 } else {
 $level = 1;
 }
 
 if ($level >= 0) {
 foreach ($data as $key => $val) {
 if ($val['pid'] == $pid) {
 $arr[$level + 1][] = $val['id'];
 $this->getTree($data, $val['id'], $level, $bool_level);
 }
 }
 }
 
 return $arr;
 
 }