php 字符串式非关联数组循环

$xiangmu='1,2,3';
$xiangmu=explode(',',$jc['xiangmu']);
$xm='';
for($i=0;$i<count($f)-1;$i++){
$xm[$i]=M('Member_card_xuanxiang')->where(array('id'=>$f[$i]))->find();
}

$this->assign('xm',$xm);