If you allow uploads on the public web, it is only a matter of time before your server is hosting and serving very illegal content. Any of the following options will prevent this:
- Don't make the folder writable by the webserver `chmod 775`
- Set `$allow_upload = false`
- Use a password `$PASSWORD = 'some password'`
- Use a `.htaccess` file with Apache, or `auth_basic` for nginx