Javascript redirect based on form input

You can't mix a meta-refresh redirect with a form submission per se. Also, meta-refreshes are terrible anyway. Since you are already in control of the receiving page, and it's using PHP, use that to accomplish the redirect. Try this:

<form action="http://site.com/page.php" method="post" name="myform">
<input type="submit" value="Go!" />
</form>

Then, in page.php:

<?php
// Act on the input, store it in the database or whatever.  Then do the redirect using an HTTP 302.
header('Location: http://example.com');
?>

If you need the form to pass the destination along to page.php, you'll want to sanitize it to prevent a LOT of security problems. Here's a rough outline.

<form action="http://site.com/page.php" method="post" name="myform">
<input type="hidden" name="destination" value="http://example.com" />
<input type="submit" value="Go!" />
</form>

Then, in page.php (copied re-encoding from answer https://stackoverflow.com/a/5085981/198299):

<?php
$destination = $_POST['destination'];
$url_parsed = parse_url($destination);
$qry_parsed = array();
parse_str($url_parsed['query'], $qry_parsed);
// Check that $destination isn't completely open - read https://www.owasp.org/index.php/Open_redirect
$query = parse_url($destination);
$destination = "{$url_parsed['scheme']}{$url_parsed['host']}{$url_parsed['path']}?" . http_build_query($query);
header('Location: ' . $destination);
?>

I haven't double-checked that code (just wrote it here in the browser), but it should suffice as a rough sketch.

in site.com/page.php

<script>window.location.href = 'newPage.php';</script>

You will have to write this outside the php tags though.

To redirect a page in PHP, use:

<?php
header('Location: url/file.php');
?>

To refresh to a different page in HTML, use:

<meta http-equiv='refresh' content='0;url=http://url/file.php'>

In the content attribute, 0 is the amount of seconds to wait.

To refresh to a different page in JavaScript, use:

window.location.href = 'url/file.php';

When none of these work, follow an anchor link, using HTML:

<a href="url/file.php">Click here to go now!</a>

To answer your question, it can be done several ways:

1) Very bad, requires two files, super redundant
HTML file:

<form action="http://site.com/page.php" method="post" name="myform">
    <input type="hidden" name="url" value="<?php=$url?>">
</form>
<script type="text/javascript">
    // Submit the form
    document.forms['myform'].submit();
</script>

Page.php:

<?php
    // Catch url's value, and send a header to redirect
    header('Location: '.$_POST['url']);
?>

2) Slightly better, still not recommended

<form action="http://site.com/page.php" method="post" name="myform">
    <input type="hidden" name="url" value="<?php=$url?>">
</form>
<script type="text/javascript">
    // Set form's action to that of the input's value
    document.forms['myform'].action = document.forms['myform'].elements['url'].value;
    // Submit the form
    document.forms['myform'].submit();
</script>

3) Still very redundant, but we're getting better

<form action="http://site.com/page.php" method="post" name="myform">
    <input type="hidden" name="url" value="<?php=$url?>">
</form>
<script type="text/javascript">
    // Simply refresh the page to that of input's value using JS
    window.location.href = document.forms['myform'].elements['url'].value;
</script>

4) Much better, save yourself a lot of trouble and just use JS in the first place

<?php
    // Start with a PHP refresh
    $url = 'url/file.php'; // Variable for our URL
    header('Location: '.$url); // Must be done before ANY echo or content output
?>
<!-- fallback to JS refresh -->
<script type="text/javascript">
    // Directly tell JS what url to refresh to, instead of going through the trouble to get it from an input
    window.location.href = "<?php=$url?>";
</script>
<!-- meta refresh fallback, incase of no JS -->
<meta http-equiv="refresh" content="0;url=<?php=$url?>">
<!-- fallback if both fail (very rare), just have the user click an anchor link -->
<div>You will be redirected in a moment, <a href="<?php=$url?>">or you may redirect right away</a>.</div>

Save that with a .php extension, and you should be good to go.