PHP short tags, which is <?, refers to short opening tag of <?php. In PHP configuration (php.ini), you can find a short_open_tag variable.

On some server/hosting provider, this option is disabled with unclear reason.

To enable this feature on your server, you will need to edit your php.ini file and change the short_open_tag setting to:

short_open_tag = On

You will have to restart your server/apache/httpd service in order to apply configuration changes.