利用php取得中文字符串中第一个汉字的字母！

<?php
$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="中文首字母A:1";
$str= iconv("UTF-8","gb2312", $str);
echo $str."</br>";
$i=0;
while($i<strlen($str) ) {
$tmp=bin2hex(substr($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;
}
?>

根据中文拼音来判断它的ascii码的编码来算！