I cannot use $_Get when I add my .htaccess - php

Question

I have a website that works fantastic. I'd like to organize it a little better so instead of loading every file by typing it in the url, i'll just make a index file that can include the files by a get method.
$key = $_GET['KEY'];
if ($key == 'signup'){
include "forming/signup.php";
}
To make the url a little cleaner I added this in the .htaccess file
RewriteEngine On
RewriteRule ^([a-zA-Z0-9]+)$ index.php?KEY=$1
RewriteRule ^([a-zA-Z0-9]+)/$ index.php?KEY=$1
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^\.]+)$ $1.php [NC,L]
Which turned
localhost:8888/index?KEY=signup
into
localhost:8888/signup
Now the only problem is that when I use the $_GET, it simply doesn't notice it. I noticed it by a redirect (Programmed in If statement) so I tested it by making a file and with this code
<?php
$test = $_GET['id];
echo "<h1>" . $test . "</h1>";
And it didn't work, like expected.
I don't know why $_GET stopped working after I used that .htaccess code.
(I'm a novice at .htaccess)
I would truly appreciate if anyone could tell me another way to cleanse the url and still have the $_GET working. (Or just if you see any error in the .htaccess)
UPDATE
I'd like to clarify. Essentially I want this
localhost:8888/index.php?KEY=post&id=1
to
localhost:8888/post/1
(But for every post, not just nr 1)

Answer 1

As PHP is a server-side language, $_GET only works on what's actually in the URL string at the time the request is sent to the server. If you purely want to alter what's visible in the address bar in order to achieve a cleaner look, you'll need to use a server-side language.
Luckily you can do this with Javascript pushState. There's an answer here that covers it in all possible detail.

Answer 2

If you want my advice, follow these steps to get the best site :)
1 -> First, create folder in your public_html , name it : "auth" for example
in this folder you can upload you files like "signin.php" , "signup.php" , "forget_password.php"
2 -> in your htaccess file , put this :
I've written this file for you, he is ready :)
<IfModule mod_rewrite.c>
<IfModule mod_negotiation.c>
deny from 10.0.0.182
AddDefaultCharset utf-8
Options -MultiViews
Options -Indexes
DirectoryIndex index.php
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*)$ $1.php
RewriteCond %{THE_REQUEST} ^(.+)\.php([#?][^\ ]*)?\ HTTP/
RewriteRule ^(.+)\.php$ /$1 [R=301,L]
#REDIRECTS HERE LIKE REF SYSTEM
RewriteRule ^ref/(.*)$ auth/signup?ref=$1 [L]
</IfModule>
</IfModule>
3 -> Your links now is :
http://example.com/auth/signin
http://example.com/auth/signup
http://example.com/auth/forget_password
http://example.com/ref/username
NOTE : in ref system , you can get username value from $_GET['ref']