案例，在做递归无限级子类的时候，在foreach循环里，静态变量会每一次循环都累加起来，如果需要每一次循环是一类的话，就需要每次循环都销毁一次静态变量

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;

}