Test plugin for blog post at http://snippets.khromov.se/useful-wordpress-filters-drew-jaynes-wordpress-talk

wordpress-test-plugin.php

<?php
/*
Plugin Name: Test plugin
Plugin URI: http://snippets.khromov.se/useful-wordpress-filters-drew-jaynes-wordpress-talk
Description: Test plugin
Version: 1.0
Author: khromov
Author URI: http://snippets.khromov.se
License: GPL2
*/

//Change the “Enter title here” placeholder text for new posts
function enter_title_filter($title, $post) {
    return __('Your own enter title here text');
}
add_filter('enter_title_here', 'enter_title_filter', 10, 2);

//Change the post updated, published, trashed messages
function post_updated_messages_filter($messages) {
	//Get the current post
	global $post;
	$post_url = get_permalink($post->ID);

	//print_r($messages); <- in case you want to check content of $messages

	$messages['post'][1] = __("Your own post updated message! Woo!") . " <a href='{$post_url}'>". __('View post') ."</a>";
	return $messages;
}
add_filter('post_updated_messages', 'post_updated_messages_filter');

//Filter content of the post editor
function post_editor_filter($content) {
	if($content == '')
		return _('Adding my own content for new posts!') . $content;
	else
		return $content;
}
add_filter('the_editor_content', 'post_editor_filter');

//Rearrange admin menu order
//Move the settings menu to the top, just because we can!
function rearrange_admin_menu_filter($menu_order) {
	//Unset current position of options menu
	foreach($menu_order as $key => $value) 	{
		if($value == 'options-general.php');
			unset($menu_order[$key]);
	}
	//Add the Options menu again at the top
	array_unshift($menu_order, 'options-general.php');

	return $menu_order;
}
add_filter('custom_menu_order', '__return_true');
add_filter('menu_order', 'rearrange_admin_menu_filter', 10, 2);

//Hide admin meta boxes by default
function change_default_hidden( $hidden, $screen ) {
	//This is the <div id="X"> value in the admin HTML code.
	$hidden[] = 'postimagediv';
	return $hidden;
}
add_filter( 'default_hidden_meta_boxes', 'change_default_hidden', 10, 2 );

//Modifying post content on the frontend
function the_content_filter($content) {
	return __('<strong>Prepending stuff to the_content! </strong>') . $content;
}
add_filter('the_content', 'the_content_filter', 10, 1);

//Adding classes to body and post wrappers
function body_class_filter($classes) {
	$classes[] = 'my-class';
	return $classes;
}
add_filter('body_class', 'body_class_filter', 10, 1);

//Changing the length of post excerpts
function post_excerpt_filter($number) {
	echo "wat";
	return 20;
}
add_filter('excerpt_length', 'post_excerpt_filter', 10, 1);

//Modifying the request
function request_filter($query_vars) {
	//Warning: Will redirect every request to a page with the slug sample-page
	//$query_vars['pagename'] = 'sample-page';
	return $query_vars;
}
add_filter('request', 'request_filter', 10, 1);