I know, that the first line in PHP Files is because of security reasons. I read it here: https://docs.joomla.org/Why_do_most_of_the_Joomla!_PHP_files_start_with_defined(%27_JEXEC%27)%3F
But in the Joomla Core Code I find different commands. For example:
\defined('JPATH_PLATFORM') or die;,
\defined('JPATH_BASE') or die;
\defined('_JEXEC') or die;
Which line should PHP files in Joomla start with?