If you are worried about access to this script you can add a random string to the command line argument of the cron job string e.g.
php /path/to/script.php random_string
Then in your script.php you check argv0 to see if that string is exists and is correct before running the rest of your code.
Another way is to put the cron script in a folder and restrict its access using a .htaccess file. Here is an answer on how to do this: https://serverfault.com/questions/92192/apache-htaccess-file-to-restrict-access-to-all-files-unless-local-ip