php登陆界面刷新验证码 javascript 的写法

<script type="text/javascript">
    function refreshVerify(){
        var imgId = document.getElementById("getVerify");
        imgId.src = "getVerify.php?ran=" + Math.random();
    }
</script>

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<a href='javascript:refreshVerify();'>
<img id='getVerify' src="getVerify.php" alt="" title="点击刷新"/>
</a>