javascript 自定义函数 为什么不能执行

javascript 自定义函数   为什么不能执行
<?php
/*
* Created on 2013-7-28
*
*/

include("conn.php");

     //判断是否提交
    if(!empty($_POST['sbmt'])){

    echo $sql="insert into message (id,username,title,content,lastdate) values('','$_POST[username]','$_POST[title]','$_POST[content]',now())";
    $result = mysql_query($sql);
        if($result){
            echo "<script type='text/javascript'>alert('留言成功！');</script>";   
                }else{
            echo "<script type='text/javascript'>alert('留言失败！');</script>";
        }
            echo "<script type='text/javascript'>location.href='add.php'</script>";
    }
    include("head.php");
?>
<script language='javascript'>
    function CheckPost(){
        if(myform.username.value==""){
            alter("please fill into username");
            myform.username.focus();
            return false;
        }
        if(myform.title.value.length<5){
            alter("title must be great to 5 characters");
            myform.title.focus();
            return false;
        }            
    }
</script>
<form action="add.php" ,name="myform" method="post" onsubmit="return CheckPost();">
    用户名：<input type="text" name="username" /><br/>
     标&nbsp;&nbsp;题：<input type="text" name="title" /><br/>
    留言内容：<textarea name="content" cols="60" rows="2"></textarea><br/>
    <input type="submit" name="sbmt" value="留言" /><br/>
</form>