求教，如何判断是否是低字节？

<?phpheader("content-type:text/html;charset=gbk");$str="j这是ces测试dkfjdksj";function substr_gbk($str,$start,$end){	$temp="";	for($i=$start;$i<=$end;$i++)	{		if(ord($str{$i})>0xa0)		{			$temp.=substr($str,$i,2);			$i++;		}else		{			$temp.=substr($str,$i,1);		}	}	return $temp;}echo substr_gbk($str,1,10); //从高字节截取正常echo substr_gbk($str,2,10); //从低字节截取出现乱码?>

$str="j这是ces测试dkfjdksj"; function substr_gbk($str,$start,$end){    $temp="";    for($i=0; $i<$start; $i++) if(ord($str{$i})>0xa0) $i++; //加上这行    for(;$i<=$end;$i++)    {        if(ord($str{$i})>0xa0)        {            $temp.=substr($str,$i,2);            $i++;        }else        {            $temp.=substr($str,$i,1);        }    }    return $temp;} echo substr_gbk($str,1,10); //这是ces测试echo substr_gbk($str,2,10); //是ces测试

<?phpheader("content-type:text/html;charset=gbk");$str="c这是ces测试dkfjdksa";function substr_gbk($str,$start,$end){	for($i=0;$i<strlen($str);$i++)	{		if(ord(substr($str,$i,1))>0xa0)		{			$temp[]=substr($str,$i,2);			$i++;		}else		{			$temp[]=substr($str,$i,1);		}	} 	return implode("",array_slice($temp,$start,$end));}print_r(substr_gbk($str,0,3));?>

<?phpheader("content-type:text/html;charset=gbk");$str="这是ces测试dkfjdksa";echo mb_substr($str,1,5); //乱码?>