JavaScript checkbox submit only one input

I have problem on the checkbox submission. The code is as below: (1)

<script type="text/javascript">
function init(){
document.getElementById("pizza").topping[0].checked=true;
document.getElementById("btn").onclick=poll;


}
onload=init;

function poll(){
var i, isOK, summary="";
var form=document.getElementById("pizza");
for(i=0;i<form.topping.length;i++)
{
        if(form.topping[i].checked)
        {summary+=form.topping[i].value+" ";}
}

isOK=confirm("Submit these choices?\n"+summary);
if(isOK){form.submit();}else{return false;}

}
</script>
<form id="pizza" action="poll.php" method="POST">
        <div id="panel">Pizza Topping?
                <input type="checkbox" name="topping" value="Cheese">Cheese</input>
                <input type="checkbox" name="topping" value="Ham">Ham</input>
                <input type="checkbox" name="topping" value="Peppers">Peppers</input>
                <input id="btn" type="button" value="Confirm Choices"></input>
        </div>
</form>

poll.php:

<?php
echo "<br>***************<br>";var_dump($_POST);

echo '*********************';
?>

The JS confirm message: Submit these choices? Cheese Peppers

The output of poll.php:

---

array (size=1) 'topping' => string 'Peppers' (length=7)

---

So my problem is: I checked Cheese and Peppers but I only got Peppers submitted. Can anyone help me?