How do I get a Function in my functions.php to execute with a cron job?

Question

When you use *nix cron to run a PHP file like that it won’t have WordPress loaded. You can load it manually, but a better method is to use WordPress’ own cron API.

Schedule the event in your functions.php:

if ( ! wp_next_scheduled( 'expire_posts' ) ) {
    wp_schedule_event( time(), 'hourly', 'expire_posts' );
}

I’ve used hourly, the other available defaults are daily & twicedaily (more on scheduling).

Now let’s hook up your expire_posts function to our newly created expire_posts event:

add_action( 'expire_posts', 'expire_posts' );

Tada!

NB: WordPress cron relies on a request (i.e. a user or bot visiting the site) at least as often as your schedule to run smoothly. If your site is quiet we can use *nix cron to trigger the WordPress cron API every 15 minutes and let WordPress handle its own internal events.

First, disable the default request-based cron behaviour in wp-config.php:

define( 'DISABLE_WP_CRON', true );

Now set up a *nix cron to trigger WordPress’ cron:

/15 * * * wget -q -O - http://yourdomain.com/wp-cron.php?doing_wp_cron