中文字符串分割

/**
 * Convert a string to an array
 * @param string $str
 * @param number $split_length
 * @return multitype:string
 */
function mb_str_split($str,$split_length=1,$charset="UTF-8"){
    if(func_num_args()==1){
        return preg_split('/(?<!^)(?!$)/u', $str);
    }
    if($split_length<1)return false;
    $len = mb_strlen($str, $charset);
    $arr = array();
    for($i=0;$i<$len;$i+=$split_length){
        $s = mb_substr($str, $i, $split_length, $charset);
        $arr[] = $s;
    }
    return $arr;
}