PHP gets the first letter of Chinese string

$limit = array( //gb2312 拼音排序
    array(45217,45252), //A
    array(45253,45760), //B
    array(45761,46317), //C
    array(46318,46825), //D
    array(46826,47009), //E
    array(47010,47296), //F
    array(47297,47613), //G
    array(47614,48118), //H
    array(0,0),         //I
    array(48119,49061), //J
    array(49062,49323), //K
    array(49324,49895), //L
    array(49896,50370), //M
    array(50371,50613), //N
    array(50614,50621), //O
    array(50622,50905), //P
    array(50906,51386), //Q
    array(51387,51445), //R
    array(51446,52217), //S
    array(52218,52697), //T
    array(0,0),         //U
    array(0,0),         //V
    array(52698,52979), //W
    array(52980,53688), //X
    array(53689,54480), //Y
    array(54481,55289), //Z
);

$str="A:这是一个测试程序1";
$str= iconv("UTF-8","gb2312", $str);
echo $str."";
$i=0;
while($i    $tmp=bin2hex(substr($str,$i,1));
    if($tmp>='B0'){ //汉字的开始
        $t=getLetter(hexdec(bin2hex(substr($str,$i,2))));
        printf("%c",$t==-1 ? '*' : $t );
        $i+=2;
    }
    else{
        printf("%s",substr($str,$i,1));
        $i++;
    }
}

function getLetter($num){
    global $limit;
    $char_index=65;
    foreach($limit as $k=>$v){
        if($num>=$v[0] && $num<=$v[1]){
            $char_index+=$k;
            return $char_index;
        }
    }
    return -1;
}
?>

首字母, PHP