Plugin Defaults

Action Hooks

<?php
add_action ( 'hook_name', 'your_function_name', [priority],
[accepted_args] );

Adding text after each item's content

<php 
add_filter( 'the_content', 'ch2epl_email_page_filter' );

function ch2epl_email_page_filter ( $the_content ) {
  // build url to mail message icon downloaded
  // from iconarchive.com
  $mail_icon_url = plugins_url( 'mailicon.png', __FILE__ );
  // Set initial value of $new_content variable to previous
  // content
  $new_content = $the_content;
  // Append image with mailto link after content, including
  // the item title and permanent URL
  $new_content .= "<a title='E-mail article link'
  href='mailto:[email protected]?subject=Check out
  this interesting article entitled ";
  $new_content .= get_the_title();
  $new_content .= "&body=Hi!%0A%0AI thought you would enjoy
  this article entitled ";
  $new_content .= get_the_title();
  $new_content .= ".%0A%0A";
  $new_content .= get_permalink();
  $new_content .= "%0A%0AEnjoy!'>
  <img alt='' src='" . $mail_icon_url. "' /></a>";
  // Return filtered content for display on the site
  return $new_content;
}

Google Analytics

<?php
add_action( 'wp_head', 'ch2pho_page_header_output' );
function ch2pho_page_header_output() { ?>
<script type="text/javascript">
var gaJsHost = ( ( "https:" == document.location.protocol ) ?
"https://ssl." : "http://www." );
document.write( unescape( "%3Cscript src='" + gaJsHost +
"google-analytics.com/ga.js' \n\
type='text/javascript'%3E%3C/script%3E" ) );
</script>
<script type="text/javascript">
try {
var pageTracker = _gat._getTracker( "UA-xxxxxx-x" );
pageTracker._trackPageview();
} catch( err ) {}
</script>
<?php }

Header Plugin

<?php
/*
Plugin Name: Chapter 2 - Plugin Header
Plugin URI:
Description: Declares a plugin that will be visible in the
WordPress admin interface
Version: 1.0
Author: Yannick Lefebvre
Author URI: http://ylefebvre.ca
License: GPLv2
*/
?>

Inserting link statistics tracking code in page body

<?php 
add_filter( 'the_content', 'ch2lfa_link_filter_analytics' );
function ch2lfa_link_filter_analytics ( $the_content ) {
  $new_content = str_replace( "href",
  "onClick=\"recordOutboundLink(this, 'Outbound Links', '" .
  home_url() . "' );return false;\" href", $the_content );
  return $new_content;
}


add_action( 'wp_footer', 'ch2lfa_footer_analytics_code' );

function ch2lfa_footer_analytics_code() { ?>
  <script type="text/javascript">
  function recordOutboundLink( link, category, action ) {
  _gat._getTrackerByName()._trackEvent( category, action );
  setTimeout( 'document.location = "' + link.href + '"', 100 );
  }
  </script>
  <?php }

Loading External Files

<?php 
add_action( 'wp_head', 'ch2fi_page_header_output' );
function ch2fi_page_header_output() {
  $icon_url = plugins_url( 'favicon.ico', __FILE__ );
  ?>
  <link rel="shortcut icon" href="<?php echo $icon_url; ?>" />
  <?php }
  
//Other Functions to Find File Locations
// get_theme_root(): Returns the address of the theme installation directory
//  get_template_directory_uri(): Retrieves the URI to the current theme's files
//  admin_url(): Provides the address of the WordPress administrative pages
//  content_url(): Indicates where the wp-content directory can be found
//  site_url() and home_url(): Return the site address
// includes_url(): Provides the location of WordPress include files
//  wp_upload_dir(): Indicates the directory where user-uploaded files are stored

Modifying the page title

<?php
add_filter( 'wp_title', 'ch2tf_title_filter' );

function ch2tf_title_filter ( $title ) {
//Select new title based on item type
  if ( is_front_page() )
  $new_title = 'Front Page >> ';
  elseif ( get_post_type() == 'page' )
  $new_title = 'Page >> ';
  elseif ( get_post_type() == 'post' )
  $new_title = 'Post >> ';
  // Append previous title to title prefix
  if ( isset( $new_title ) ) {
  $new_title .= $title;
  // Return new complete title to be displayed
  return $new_title;
  } else {
  return $title;
  }
}

//Output: the browser's title bar or navigation tabs display additional text before
//their name

Troubleshooting coding errors and printing variable content

<?php 
//inside wp-config
define( 'WP_DEBUG', true );

//Other debugging features that can be activated from the wp-config.php file are as follows:
// WP_DEBUG_LOG: Stores all debug messages in a file named debug.log in the site's
//wp-content directory for later analysis
//  WP_DEBUG_DISPLAY: Indicates whether or not error messages should be
//  displayed on-screen
//  SAVEQUERIES: Stores database queries in a variable that can be displayed in the
//  page footer (see http://codex.wordpress.org/Editing_wp-config.
//  php#Save_queries_for_analysis for more info)