[安洵杯 2019]easy_serialize_php

<?php

$function = @$_GET["f"];

function filter($img){
    $filter_arr = array("php","flag","php5","php4","fl1g");
    $filter = "/".implode("|",$filter_arr)."/i";
    return preg_replace($filter,"",$img);
}


if($_SESSION){
    unset($_SESSION);
}

$_SESSION["user"] = "guest";
$_SESSION["function"] = $function;

extract($_POST);

if(!$function){
    echo "<a href="index.php?f=highlight_file">source_code</a>";
}

if(!$_GET["img_path"]){
    $_SESSION["img"] = base64_encode("guest_img.png");
}else{
    $_SESSION["img"] = sha1(base64_encode($_GET["img_path"]));
}

$serialize_info = filter(serialize($_SESSION));

if($function == "highlight_file"){
    highlight_file("index.php");
}else if($function == "phpinfo"){
    eval("phpinfo();"); //maybe you can find something in here!
}else if($function == "show_image"){
    $userinfo = unserialize($serialize_info);
    echo file_get_contents(base64_decode($userinfo["img"]));
}