php 过滤字符串中的中文和数字

function getChinese($string,$encode="GBK") {
       switch($encode){
            case "GBK" :$codelength=2;break;
       case "GB2312" :$codelength=3;break;
       case "UTF-8" :$codelength=3;break;
           case "UTF-16" :$codelength=4;break;

       }
       $tmpstr = '';
       $arr = array(1,2,3,4,5,6,7,8,9,0);
       $strlen = strlen($string);
       for($i=0; $i<$strlen; $i++) {
            $str=substr($string, $i, 1);
            $str1=trim($str);
            if( ord($str)>0xA0 ){
            $tmpstr.= substr($string, $i, $codelength);
            $i = $i+$codelength-1;
            }

}
    return $tmpstr;
}
//该片段来自于http://outofmemory.cn