PHP文本框的值随下拉框改变

初学PHP，下面是实现文本框内容随下拉框变化的代码实现：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>PHP文本框的值随下拉框改变</title>
<script type="text/javascript">
var m = "5";
document.getElementById("test").value= m;

</script>

<script language="javascript">
function look(){
  var se =document.getElementById("sel");
  var option=se.getElementsByTagName("option");
  var str = "" ;
  for(var i=0;i<option.length;++i)
  {
  if(option[i].selected)
  {
  document.getElementById("test").value = option[i].text;
  }
  }
}

</script>
</head>

<body>

<input type="text" name="test" id="test">

<select name="sel" id="sel" onchange="look();">
<option value="1" >11</option>
<option value="2" selected>22</option>
<option value="3">33</option>
</select>
</body>
</html>