表单 - 从客户端提交到PHP：字段序列是否可靠？

I am working on a very complex form that can be completly customized by the user with JavaScript. This can end up in hundreds of fields (including a lot of hidden meta information).

I would like to know if there is any way the correct sequence of form elements (their sequence within the HTML code) can be corrupted on their way from "client browser A" (e.g. Firefox) to "server-side scripting language B" (e.g. PHP).

W3C only states: "A form data set is a sequence of control-name/current-value pairs..." which leads me to assume that this sequence should be persistent during the whole form post process.

Can I completely rely on the correctness of the field sequence when submitting from any browser to (in my case) PHP?

Or do I have to double check?

In case my question is not clear, here is a simplified example.

1) the server delivers this form:

<input type="text" name="servergenerated_formfield_1" value="position 1">
<input type="text" name="servergenerated_formfield_2" value="position 2">
<input type="text" name="servergenerated_formfield_3" value="position 3">

2) the user creates 2 new form elements using Javascript:

<input type="text" name="USERGENERATED_FORMFIELD_1" value="lorem ipsum">
<input type="text" name="servergenerated_formfield_1" value="position 1">
<input type="text" name="servergenerated_formfield_2" value="position 2">
<input type="text" name="USERGENERATED_FORMFIELD_2" value="dolor sit amet">
<input type="text" name="servergenerated_formfield_3" value="position 3">

3) Now when I access the array (using PHP):

foreach ($_POST as $key => $value)
{
    echo $key.': '.$value.'<br>';
}

... is there ANY chance I might end up with something ELSE than this?

// output
USERGENERATED_FORMFIELD_1: lorem ipsum
servergenerated_formfield_1: position 1
servergenerated_formfield_2: position 2
USERGENERATED_FORMFIELD_2: dolor sit amet
servergenerated_formfield_3: position 3