php 生成唯一随机码

/**
 * 创建邀请码
 * @param $length
 * @return array
 * @author Mr lj
 */
function createInvitationCode($num=1,$length=8)
{
    $sign = ['_','#','$','&','*','-'];
    $mixedArr = array_merge(range('a','z'),range('A','Z'),range(0,9),$sign);
    $data = [];
    $i = 0;
    $str = '';

    for($a=0;$a<$num;$a++)
    {
        while($i < $length)
        {
            $key = mt_rand(0,count($mixedArr)-1);
            $str .= $mixedArr[$key];
            ++$i;
        }
        $res = M('SystemCode')->where(['code'=>$str])->find();

        (($res == true) && --$a) || $data[] = $str;

        $str = '';
        $i = 0;
    }

    return $data;
}