Ask for advice, how to judge whether it is low byte

Ask, how to determine whether it is low-byte?

 
  Header ("CONTENT-TYPE:TEXT/HTML;CHARSET=GBK");
$str = "J This is CES test 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 ($str, 1,10); Intercept normal from high byte
Echo SUBSTR_GBK ($str, 2,10); Garbled truncation from low byte
?>

The above code is truncated from high byte without problem, but if truncation from low byte is garbled
Do you know how to determine the high and low byte method?

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------Solution--------------------
This post was last edited by xuzuning on 2013-10-30 11:08:53

$str = "J This is CES test dkfjdksj";

function SUBSTR_GBK ($str, $start, $end)
{
    $temp = "";
    for ($i =0; $i < $start; $i + +) if (Ord ($str {$i}) >0xa0) $i + +; Plus this line.
    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 ($str, 1,10); This is the CES test.
Echo SUBSTR_GBK ($str, 2,10); It's a CES test.

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