Come ha giustamente detto Daniele, il motivo è il tag di apertura.
Dalla documentazione ufficiale di PHP:
When PHP parses a file, it looks for opening and closing tags, which are <?php and ?> which tell PHP to start and stop interpreting the code between them.
[...]
PHP also allows for short open tag <? (which is discouraged since it is only available if enabled using the short_open_tag php.ini configuration file directive, or if PHP was configured with the --enable-short-tags option).