PHP“动态增加文本框”!!

Question

<script language="javascript">function filesnum(num){
var str="";
for(i=0;i<num;i++){
str += '<input name="files[]" type="file"">\n';
}
filestd.innerHTML = str;
}
</script>
<input name="radiobutton" type="radio" onClick="filesnum(1)" value="radiobutton" checked>
1
<input type="radio" name="radiobutton" value="radiobutton" onClick="filesnum(2)">
2
<input type="radio" name="radiobutton" value="radiobutton" onClick="filesnum(3)">
3
<input type="radio" name="radiobutton" value="radiobutton" onClick="filesnum(4)">
4
<input type="radio" name="radiobutton" value="radiobutton" onClick="filesnum(5)">
5 <br>
<div id="filestd">
  <input name="files[]" type="file">
</div>