Run a script.php on cron job on linux/apache server but restrict public access to the php file

I have this script.php file which i want to run as a cron job on my linux/apache server.

However, i do not want public to access www.mycompanyname.com/script.php and also run the script concurrently.

How can we prevent that? How can we restrict the script to the server's access only? Is it using chmod or setting something inside .htaccess file, something along the line?

Any advice ?