A simple sharex custom upload script

sharex.php script

<?php
#No dashes in the end/begin of directory or domain
$key = "key_here";
$file_dir = "i"; #Make sure this does exist already
$nameLength = 5;
$IMGonly = true;
$domain = 'domain_here';

if (isset($_POST['key'])){
    if ($_POST['key'] == $key){
        if ($IMGonly == true && is_array(getimagesize($_FILES["file"]["tmp_name"])) == true || $IMGonly == false) {
            $target = $file_dir.'/'.createID($nameLength).'.'.pathinfo($_FILES["file"]["name"], PATHINFO_EXTENSION);
            if (move_uploaded_file($_FILES["file"]["tmp_name"], $target)){
                echo 'https://'.$domain.'/'.$target;
            } else {
                echo "File upload failed. Does directory ".$file_dir." exist?";
            }
        } else {
            echo "File not accepted!";
        }
    }else {
    echo "key not valid";
    }
} else {
    echo "No key specified";
}

function createID($length) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $charactersLength = strlen($characters);
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }
    return $randomString;
    }
?>