Use a function to update post meta based on other post meta

Here are two simple ways to do this:

One is to use the the_content-filter to update the post “hotness”, or you can schedule an event that will run daily and update the “hotness” once a day.

First approach: the_content-filter

Turn you code in to a “callable” function:

function update_hotness( $post_id, $echo = false ) {
    $time_ago = human_time_diff( get_the_time( 'U', $post_id ), current_time('timestamp') );
    if ( strtotime($time_ago) <  strtotime('1 day') ) {
        $time_ago = "1";
    }

    $days_ago = preg_replace( "/[^0-9 ]/", '', $time_ago );
    $days_ago;
    $ratings_score = get_post_meta( $post_id, 'ratings_score', true );
    $hotness = $ratings_score / $days_ago;
    // Here you store the "hotness" as post meta
    update_post_meta( $post_id, 'hotness', $hotness );
    if ( $echo )
        echo $hotness;
    return $hotness;
}

So you can now get and update the post “hotness” with this function by just passing it the post id.

Next you create a hooked function to the_content-filter which will update the posts “hotness” every time the_content is called:

add_filter( 'the_content','update_hotness_filter' );
function update_hotness_filter( $content ) {
    global $post;
    update_hotness( $post->ID );
    return $content;
}

Second approach: Scheduling an event

First, create a function that will check if your event is scheduled or not, then update accordingly:

add_action( 'wp', 'hotness_update_activation' );
function hotness_update_activation() {
    if ( ! wp_next_scheduled( 'daily_hotness_update' ) ) {
        wp_schedule_event( current_time( 'timestamp' ), 'daily', 'daily_hotness_update' );
    }
}

Finally you make sure to add an action hook to your event and a function that will run every day:

add_action( 'daily_hotness_update', 'daily_hotness_update_callback' );
function daily_hotness_update_callback() {
    // Here you get a list of posts to update the "hotness" for.
    // Then you just call `update_hotness()` for each one ex:
    foreach ( $posts as $p ) {
        update_hotness( $p->ID );
    }
}

And that is it 🙂

If you ask me you should go with the simpler way, which is the first approach.