/**
* 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;
}