mod_rewrite redirect loop / breaks my site - php

Question

What I want:
/foo.php display /foo in url bar
/foo redirect to /foo.php
Both must view /foo.php
Must work with /showthread.php?id=00 (/showthread?id=00) and such
I constantly get a redirect loop even if I use [L] everywhere.
I tried these:
(1)
RewriteRule ^/forums$ forums.php [R]
RewriteRule ^/forums\.php$ forums [PT]
(2)
RewriteRule ^forums\.php/?$ forums [L,R=301]
RewriteRule ^forums/?$ forums.php
(3)
RewriteRule ^forums/?$ forums.php [L,R=301]
(4)
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/?([a-z/]+).php$ $1 [R,L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.*) $1.php [L]
Each of the above work for individually foo.php to foo or foo to foo.php but infinite loop on both.
I also tried this:
## don't touch /forum URIs
# RewriteRule ^forums/ - [L,NC]
## hide .php extension snippet
# To externally redirect /dir/foo.php to /dir/foo
# RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.php [NC]
# RewriteRule ^ %1 [QSA,R,L]
# To internally forward /dir/foo to /dir/foo.php
# RewriteCond %{REQUEST_FILENAME} !-d
# RewriteCond %{REQUEST_FILENAME}.php -f
# RewriteRule ^(.*?)/?$ $1.php [QSA,L]
Which works as expected but breaks a lot of site features for some reason. (search bar does not work, forums don't work, login doesn't work)
I need foo.php to still work because MyBB uses it.

Answer 1

Try this code in root .htaccess:
RewriteEngine On
RewriteRule ^forums/ - [L,NC]
## hide .php extension snippet
# To externally redirect /dir/foo.php to /dir/foo
RewriteCond %{REQUEST_METHOD} !POST
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+)\.php [NC]
RewriteRule ^ %1 [R,L]
# To internally forward /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+?)/?$ $1.php [L]