字符串中带有&的数据转换为数组

字符串:
// 'data' => string 'pic_id=128&pic_id=124&pic_id=126' (length=32)
最终转换成:
// array (size=2)
// 0 => string '124' (length=3)
// 1 => string '126' (length=3)

        $str=$_POST['data'];
        //判断是否选中多个
        if(strpos($str, '&')){
            $a = explode('&', $str);
            foreach($a as $k=>$vo){
                $b[] = explode('=',$vo);
            }
            foreach($b as $k=>$vo){ 
                 $pic_num[] = $vo[1];
             }
        }