This is how unforgiving PHP is, if there’s an error in the code, your site can turn into a blank white page. To fix this, simply go into your server and delete the code you added.
Access to your server via FTP or File Manager (through your hosting control panel).
Apache’s .htaccess files allow per-directory configuration changes without editing the main server configuration file.
Nope. You should never change anything in the WordPress core files because all your changes will be lost during the next update. You should use actions and filters instead.