PHP - Image saver

<?php

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: POST, GET');
header('Content-type: application/json');
header('Access-Control-Allow-Headers: Content-Type');

if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {
   http_response_code(200);
}

function generate_string($strength = 32) {
    $input = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $input_length = strlen($input);
    $random_string = '';
    for($i = 0; $i < $strength; $i++) {
        $random_character = $input[mt_rand(0, $input_length - 1)];
        $random_string .= $random_character;
    }

    return $random_string;
}

if(isset($_GET['key'])) {
    $myObj->keyIsset = true;
    if($_GET['key'] === "XXXXXXXXXXXXXYOURKEYHEREXXXXXXXX"){
        if(!empty($_FILES['image'])){
            $filename = generate_string(32);
            $allowed_extensions = ['tif', 'tiff', 'bmp', 'jpg', 'jpeg', 'gif', 'png', 'jiff'];

            list(,$extension) = explode('/',$_FILES['image']['type']);
            if(in_array(strtolower($extension), $allowed_extensions)){
                $full_filename = $filename.".".$extension;

                move_uploaded_file($_FILES['image']['tmp_name'], "/var/www/img/".$full_filename);

                $full_url = "https://img.domain.net/articles/".$full_filename;
                $myObj->url = $full_url;
            } else {
                $myObj->url = 'Invalid';
            }
        }
    }
} else {
    $myObj->keyIsset = false;
}

$json = json_encode($myObj);
if ($json === false) {
    $json = json_encode(["jsonError" => json_last_error_msg()]);
    if ($json === false) {
        $json = '{"jsonError":"unknown"}';
    }

    http_response_code(500);
}

echo $json;