Fix nasty PHP FastCGI Error: No input file specified. Tested on XAMPP Windows and Debian with LAMP stack.

.htaccess

# Disable directory browsing
Options All -Indexes

# ----------------------------------------------------------------------
# Rewrite engine
# ----------------------------------------------------------------------

# Turning on the rewrite engine is necessary for the following rules and features.
# FollowSymLinks must be enabled for this to work.
<IfModule mod_rewrite.c>
	Options +FollowSymlinks
	RewriteEngine On

	# Fix PHP FastCGI Error: No input file specified.
	# Because you are lazy bastard that won't added "?" between "index.php" and "/".
	<IfModule mod_fcgid.c>
		RewriteRule .* -  [E=UNDER_FASTCGI:?]
	</IfModule>

	# If you installed CodeIgniter in a subfolder, you will need to
	# change the following line to match the subfolder you need.
	# http://httpd.apache.org/docs/current/mod/mod_rewrite.html#rewritebase
	# RewriteBase /

	# Redirect Trailing Slashes...
	RewriteCond %{REQUEST_FILENAME} !-d
	RewriteCond %{REQUEST_URI} (.+)/$
	RewriteRule ^ %1 [L,R=301]

	# Rewrite "www.example.com -> example.com"
	RewriteCond %{HTTPS} !=on
	RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
	RewriteRule ^ http://%1%{REQUEST_URI} [R=301,L]	

	# Checks to see if the user is attempting to access a valid file,
	# such as an image or css document, if this isn't true it sends the
	# request to the front controller, index.php
	# RewriteCond %{THE_REQUEST} ^/robots\.txt$ [OR]
	RewriteCond %{REQUEST_FILENAME} !-f
	RewriteCond %{REQUEST_FILENAME} !-d
	RewriteRule ^([\s\S]*)$ index.php%{ENV:UNDER_FASTCGI}/$1 [L,NC,QSA]

	# Ensure Authorization header is passed along
	RewriteCond %{HTTP:Authorization} .
	RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
</IfModule>

<IfModule !mod_rewrite.c>
	# If we don't have mod_rewrite installed, all 404's
	# can be sent to index.php, and everything works as normal.
	ErrorDocument 404 index.php
</IfModule>

<IfModule mod_headers.c>
    Header unset X-Powered-By
    Header always unset X-Powered-By
</IfModule>

# Disable server signature start
	ServerSignature Off
# Disable server signature end

This .htaccess file is a modification of CodeIgniter 4's default .htaccess. I only added these lines:

<IfModule mod_fcgid.c>
RewriteRule .* -  [E=UNDER_FASTCGI:?]
</IfModule>

Then modify the front end:

RewriteRule ^([\s\S]*)$ index.php%{ENV:UNDER_FASTCGI}/$1 [L,NC,QSA]

Tested on CodeIgniter 3 too. Likely going to work on any PHP application with FastCGI enabled and require ? between index.php and /.

Explanation:

• <IfModule mod_fcgid.c>: just other module, only execute under this tag if module is enabled.
• RewriteRule .* - [E=UNDER_FASTCGI:?]: it assigns ? into a variable ${ENV:UNDER_FASTCGI}. ${ENV:UNDER_FASTCGI} value is either ? or blank.