无法使用jQuery和php发布单选按钮的值

I'm trying to get the selected value of two radio buttons by posting via jQuery and processing it in PHP. Here's my code:

HTML:

<input type="radio" name="purch" value="goods" checked>Goods<br>
<input type="radio" name="purch" value="services">Services

jQuery:

$("#addDoc").click(function(){
    $.post("manual.php",
    {
        invoice: $("#invoiceBox").val(),
        supplierName: $("#supplierBox").val(),
        refNo: $("#refBox").val(),
        vatReg: $("#vregBox").val(),
        vatPurch: $('input[name=purch]:checked', '#addForm').val()
    },
    function(data){
      alert(data);
    });
});

PHP:

<?php
    $vatPurch = $_POST['vatPurch'];

    echo $vatPurch;
?>

However I try to post it, I always get an 'Notice: Undefined index: vatPurch' error when submitting the form.

You are using 'vatPurch' instead of 'purch'

Give a try to this and (I hope) it will work

<?php
    $vatPurch = $_POST['purch'];
    echo $vatPurch;
?>