php + javascript 验证验证码

<html>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

<body>

<form name="form1">

  <table width="300" border="1">

    <tr>

      <td width="100">验证码</td>

      <td width="100"><input type="text" name="checkcode" id="checkcode"  width="50" onblur="javascript:checknumber(this.value)"/></td>

      <td width="100"><img src="number.php" name="number" id="number" onclick="javascript:renumber()" alt="看不清，换一张" /></td>

    </tr>

    <tr>

      <td colspan="3"> <input type="text" id="showcode"></td>

    </tr>

  </table>

 

</form>

</body>

</html>

<script language="javascript" type="text/javascript">

function checknumber(msg)

{

	//怎么判断验证码是否正确？

	document.getElementById('showcode').value=msg;



}



function renumber()

{

	document.getElementById('number').src="number.php?"+ Math.random();

}

</script>