Hooks

apply_filters.php

<p><?php apply_filters( 'product_display_footer', 'Thanks for using our product display plugin' ) ?></p>

do_action.php

<h1><?php the_title() ?></h1>
<div class="description"><?php the_content() ?></div>
<?php do_action( 'below_product_description', $post ) ?>

first-filter.php

add_filter( 'login_errors', 'modify_login_errors' );
function modify_login_errors() {
    return 'Login unsuccessful, try again';
}

first-hook.php

add_action( 'wp_insert_post', 'email_post_author', 10, 3 );
function email_post_author( $post_id, $post, $update ) {
  $email = 'mymail@mail.com';
  $subject = 'New Post Published';
  $message = 'A new post was published, use this link to view it: ' . get_permalink( $post->ID );
  wp_mail( $email, $subject, $message );
}

hook-order.php

add_action( 'save_post', 'send_admin_email', 10, 3 );
add_action( 'save_post', 'update_post_stats', 5, 3 );

multiple-emails.php

add_action( 'wp_insert_post', 'send_author_email', 10, 3 );
add_action( 'wp_insert_post', 'send_admin_email', 10, 3 );

plugin.php

<?php
/**
 * Plugin Name: Hook Test
 * Description: A simple plugin to test hooks
 */

pseudo-code.php

function wp_publish_post( $post ) {
  // Stuff that actually published the post  
  do_action( 'wp_insert_post', $post->ID, $post, true );
}

wp-login.php

if ( ! empty( $errors ) ) {
	/**
	 * Filter the error messages displayed above the login form.
	 *
	 * @since 2.1.0
	 *
	 * @param string $errors Login error message.
	 */
	echo '<div id="login_error">' . apply_filters( 'login_errors', $errors ) . "</div>\n";
}