How do sites such as Stack Overflow submit forms using action="/questions/ask/submit"?

I would've thought mod_rewrite would lose the $_POST vars?

.htaccess

RewriteEngine On 
RewriteCond %{REQUEST_FILENAME} !-f 
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [L]

index.php

$q = explode("/", $_SERVER['REQUEST_URI']);
if($q[1]!=""){
  switch($q[1]){
    case "test":
      include("test.php");
      break;
      ...

test.php

<?php
  if(isset($_POST["submitButton"])){
    echo "Submitted";
  }
  else{
    echo "Not submitted";
  }
?>
<form method="post" action="/test/submit">
  <input type="submit" name="submitButton">
</form>

If I remove action="/test/submit"

If my URL is /test then it returns Not submitted when I click the button.
If my URL is /test.php then it returns Submitted when I click the button.

Update

For the time being, I'm using the following.

index.php

if($_SERVER['REQUEST_METHOD'] == 'POST'){
  $q = explode("/", $_POST["url"]);
}
else{
  $q = explode("/", $_SERVER['REQUEST_URI']);
}
...

test.php

<form method="post" action="/">
  <input type="hidden" name="url" value="/test">
  ...

This allows my test.php to receive the post vars.

If there are no errors by the user, I use header("Location: test/success");

If there are errors, the URL will have to be / which isn't ideal.

Update/resolution

The problem may be with Apache 2.4. The fix was to add this line to index.php:

parse_str(file_get_contents("php://input"),$_POST);

With this method, there's no need for any action="..." (this will successfully POST to itself, even if the URL is a slug).

mod_rewrite does not affect post variables.

action is the target of the form. method is the submit type. method would be what determines if $_POST is populated or $_GET.

EDIT

after a down vote, and some changes to the question, it seems that this response is not what the questioner is looking for.

/questions/ask/submit is probably controlled by a CMS. meaning most requests are redirected to a single file, which then interprets them. This redirect is done with mod_rewrite usually, as your question indicates. mod_rewrite accepts a number of 'switches' to tell it what to do. That being said, there is no known bug with mod_rewrite 'messing up' $_POST data. Apache is what passes this data to PHP. mod_rewrite passes that to Apache. If your $_POST data gets messed up, you have a bad rule some where.

Here is an example of how a common CMS, WordPress, directs all traffic, regardless of method, to a central controller, which then interprets the untarnished $_POST data:

# BEGIN WordPress
<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  RewriteRule ^index\.php$ - [L] 
  RewriteCond %{REQUEST_FILENAME} !-f 
  RewriteCond %{REQUEST_FILENAME} !-d 
  RewriteRule . /index.php [L] 
</IfModule>
# END WordPress

TOP DOWN EXPLANATION:

• only execute this if the mod_rewrite module is active
• turn on the rewrite engine, so that it accepts your new rules
• make all your rewrites, rewrite to the document root /
• if the request was directly to index.php then force mod_rewrite to not process more rules and load index.php
• if the requested url does not directly map to a file
• AND if the requested url does not direclty map to a directory
• send all data, including requested url, _GET, _POST, etc, to index.php, and dont process any other rules

EDIT (after sample files were submitted in question)

Here are my complete files. With these exact files, on a Apache 2.2 and PHP 5.4 install, this works perfectly.

.htaccess

# BEGIN WordPress
<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  RewriteRule ^index\.php$ - [L]
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule . /index.php [L]
</IfModule>
# END WordPress

index.php

<?php
$q = explode("/", $_SERVER['REQUEST_URI']);
if($q[1]!=""){
  switch($q[1]){
    case "test":
      include("test.php");
    break;
  }   
}   

test.php

<?php
  if(isset($_POST["submitButton"])){
    echo "Submitted";
  }else{
    echo "Not submitted";
  }
?>
<form method="post" action="/test/submit">
  <input type="submit" name="submitButton">
</form>

directory list, so you can compare permissions

NOTE: apache runs as nobody.nobody on my test machine

drwxrwxr-x 2 nobody git 4096 Dec 22 03:34 ./
drwxrwxr-x 3 nobody git 4096 Dec 22 03:26 ../
-rw-rw-r-- 1 nobody git  244 Dec 22 03:29 .htaccess
-rw-rw-r-- 1 nobody git  134 Dec 22 03:27 index.php
-rw-rw-r-- 1 nobody git  194 Dec 22 03:28 test.php

If this does not work for you, you probably have a permission problem. This exact code works.