Simple File Upload

upload.php

<?php
$target_dir = "uploads/".date("Y-m-d");
if(!file_exists($target_dir)) {
    mkdir($target_dir);
}
$maxfilesize = 200; //MB
$filetype = ".".pathinfo($_FILES["files"]["name"], PATHINFO_EXTENSION);
$target_file = $target_dir . "/[" . date("h.m.s") . "]" . random_str(2) . "_" . $_FILES["files"]["name"];
$key = "Qvnpk7wVaYqHDEqkSCw31YWUd7bmNVaJbwsokX9aqzGDlhMb4a";
$postkey = isset($_POST["key"]) ? stripslashes(trim($_POST["key"])) : null;
$getkey = isset($_GET["key"]) ? stripslashes(trim($_GET["key"])) : null;
header('Content-Type: application/json');
if($getkey === $key) {
    echo json_encode([
        'Name' => $_SERVER[HTTP_HOST],
        'DestinationType' => 'None',
        'RequestUrl' => $_SERVER[HTTP_HOST]."/".$_SERVER['PHP_SELF'],
        'RequestType' => 'POST',
        'FileFormName' => 'files',
        'Arguments' => [
            'key' => $key
        ],
        'ResponseType' => 'Json',
        'URL' => '$JSON:[\'url\']$'
    ], JSON_PRETTY_PRINT);
    die();
}
//Check Filesize
if(filesize($_FILES["files"]["temp_name"]) >= (pow(1024,2) * $maxfilesize)) {
    echo json_encode(["error" => "File is to big."], JSON_PRETTY_PRINT);
    http_response_code(403);
    die();
}
if($target_file == null) {
    echo json_encode(["error" => "No File has been detected."], JSON_PRETTY_PRINT);
    http_response_code(403);
    die();
}
if($postkey != null && $postkey === $key) {
    if(file_exists($target_file)) {
        echo json_encode(['url' => "http://".$_SERVER[HTTP_HOST]."/".$target_file], JSON_PRETTY_PRINT);
    } else {
        move_uploaded_file($_FILES["files"]["tmp_name"], $target_file);
        echo json_encode(['url' => "http://".$_SERVER[HTTP_HOST]."/".$target_file], JSON_PRETTY_PRINT);
    }
} else {
    echo json_encode(["error" => "Wrong Key."], JSON_PRETTY_PRINT);
    http_response_code(405);
    die();
}





function random_str($length, $keyspace = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
{
    $str = '';
    $max = mb_strlen($keyspace, '8bit') - 1;
    for ($i = 0; $i < $length; ++$i) {
        $str .= $keyspace[random_int(0, $max)];
    }
    return $str;
}