Well, I suppose you can write out the query if you really want to, but it's probably not a good idea.
With PHP (or any other script), the password to the DB is stored in a script file that you can easily control the permissions of to deny anyone the ability to sniff your password. With cron, the password will be in plain view everytime the job runs, so it's possible someone will steal it.
Also, if you find yourself needing to do some sort of other maintenance job, it's a lot easier to encapsulate all of your maintenance into a single script than to put it into a bunch of separate cron jobs. You may never need to add other queries, but using a script is a very low cost to make your work more futureproof.