<form method="post" name="login" action="">
  驗 證 碼：<input type="text" name="code" class="text code"  /> <img src="code.php" id="code" />

</form>

 <img src="code.php" id="code" onclick="javascript:this.src='code.php?tm='+Math.random()" />

<script type="text/javascript" src="js/code.js"></script>

    //外部文件code.js設置局部刷新函數

   function code () {
     var code = document.getElementByIdx_x_x('code');
     code.onclick = function () {
       this.src='code.php?tm='+Math.random();
     };
   };

<form method="post" name="login" action="">
  驗 證 碼：<input type="text" name="code" class="text code"  /> <img src="code.php" id="code" />

            <span id="update">點擊刷新驗證碼</span>

</form>

<img src="code.php" id="code"/><span id="update" onclick="javascript:code.src='code.php?tm='+Math.random()"></span>

<script type="text/javascript" src="js/code.js"></script>

    //外部文件code.js設置局部刷新函數

   function code () {
     var code = document.getElementByIdx_x_x('code');

     var update = document.getElementByIdx_x_x('update');
     update.onclick = function () {
       code.src='code.php?tm='+Math.random();
     };
   };