PHP_SELF和Javascript setAttribute

Question

I have the following code in PHP:

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    <?php $value = $_POST['Field1'] ?>
    <input type="text" id="Field1" name="Field1" value="<?php echo $value ?>">
    <input type="button" value="ButtonValue" onclick="SubmitFunction()">
</form>

And some function in JavaScript:

<script>
    function SubmitFunction(){
        //... do some stuff here
        //But here I need to assign value to Field1:
        document.getElementById("Field1").setAttribute("value", "Some value");
        document.form.submit();
    }
</script>

When this code loads for the first time, it says there is undefined index Field1. It's absolutely right.

But when I set attribute value to this field using SubmitFunction(), page reloads and it still can't find Field1 ! This is my problem.

I noticed that in pure HTML code the initial form looks like:

<form>
    <input type="text" id="Field1" name="Field1" value>
    <input type="button" value="ButtonValue" onclick="SubmitFunction()">
</form>

Where is my problem (except brain)?

Answer 1

 <script>
        function SubmitFunction(){
            //... do some stuff here
            //But here I need to assign value to Field1:
        document.getElementById("Field1").setAttribute("value", "Some value");
        document.form.submit(document.getElementById("Field1").getAttribute("value"));
        }
    </script>

    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    <?php $value = $_POST['Field1'] ?>
        <input type="text" id="Field1" name="Field1" value="<?php echo $value?$value:''; ?>">
        <input type="button" value="ButtonValue" onclick="SubmitFunction()">
    </form>