You need to make sure that "/usr/bin/php" is the path to PHP on your server. It is normally "/usr/bin/php" or "/usr/local/bin/php." The portion at the end of that (the > /dev/null &) tells the cron to output the messages to null (in other words, do not output anything) and the ampersand (&) tells it to run the process in the background versus the foreground. They are not needed for the command to work.