php取得字符串首字母的方法

<?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="a:这是一个测试程序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;
}
?>