Figuring Out Hooks

hook-test.php

add_action( 'save_post', 'tweet_my_mood' );
function tweet_my_mood( $post_id, $post ) {
	echo get_post_meta( $post_id, 'mood', true );
	exit();
}

our-post-class.php

add_filter( 'post_class', 'my_post_class', 10, 3 );
function my_post_class( $classes , $class, $post_id ) {
	$date = strtotime( get_post_field( 'post_date', $post_id ) );
	if( $date < strtotime( '2010-01-01 00:00:00' ) ) {
		$classes[] = 'old-post';
	}
	
	return $classes;
}

post_class.php

/**
* Filter the list of CSS classes for the current post.
*
* @since 2.7.0
*
* @param array  $classes An array of post classes.
* @param string $class   A comma-separated list of additional classes added to the post.
* @param int    $post_id The post ID.
*/

$classes = apply_filters( 'post_class', $classes, $class, $post->ID );

tweet-mood.php

add_action( 'save_post', 'tweet_my_mood' );
function tweet_my_mood( $post_id, $post ) {
	$mood = get_post_meta( $post_id, 'mood', true );
	tweet_mood( $mood );
}

wp_publish_post.php

function wp_publish_post( $post ) {
	global $wpdb;

	if ( ! $post = get_post( $post ) )
		return;

	if ( 'publish' == $post->post_status )
		return;

	$wpdb->update( $wpdb->posts, array( 'post_status' => 'publish' ), array( 'ID' => $post->ID ) );

	clean_post_cache( $post->ID );

	$old_status = $post->post_status;
	$post->post_status = 'publish';
	wp_transition_post_status( 'publish', $old_status, $post );

	/** This action is documented in wp-includes/post.php */
	do_action( 'edit_post', $post->ID, $post );
	/** This action is documented in wp-includes/post.php */
	do_action( "save_post_{$post->post_type}", $post->ID, $post, true );
	/** This action is documented in wp-includes/post.php */
	do_action( 'save_post', $post->ID, $post, true );
	/** This action is documented in wp-includes/post.php */
	do_action( 'wp_insert_post', $post->ID, $post, true );
}