WordPress Hooks And Actions – A Guide To Execution Order

What are WordPress hooks and why do they matter?

WordPress hooks allow you to tap into key points during WordPress execution to modify default behavior. Hooks act as triggers that execute functions attached to them. There are two types of hooks in WordPress:

• Actions – Execute attached functions when triggered
• Filters – Modify data passed to them before passing back to source

Understanding hooks is key for developers looking to extend WordPress functionality. Knowledge of hook execution order allows strategic placing of code for desired effect.

Real-world examples of using WordPress hooks

Some common uses of hooks include:

• Add custom data to RSS feeds using the rss2_item filter
• Modify email content with wp_mail filter before sending
• Saving custom user meta data with user_register action
• Custom post type management using register_post_type action

Action hooks vs filter hooks – knowing the difference

Action and filter hooks enable similar extensibility but behave differently:

Action Hooks

• Execute attached functions when triggered
• Functions perform desired operations
• Does not modify passed values from source
• Examples: wp_head, wp_footer, publish_post

Filter Hooks

• Modify data passed to them before passing back
• Functions expect one input, process it and return modified output
• Used extensively to alter content
• Examples: the_title, the_content, comment_text

Knowing when to use actions vs filters is vital for effective use of hooks in WordPress.

Hook execution hierarchy in WordPress

There is a distinct hierarchy in how WordPress executes hooks during a typical request:

1. muplugins_loaded – Absolute first hook triggered
2. registered_taxonomy – Register custom taxonomies
3. registered_post_type – Register custom post types
4. plugins_loaded – Plugin initialization code hooks here
5. parse_request – Triggered before handling the request
6. pre_get_posts – Alter main query variables
7. wp – Handles core WordPress functionality
8. the_post – Setup post data for display
9. wp_head – Add output inside head tag
10. wp_footer – Output before closing body tag
11. shutdown – Absolute final hook triggered

There are hundreds more hooks that fire at specific points during loading, rendering, commenting, user management etc.

Why hook order matters

Knowing execution order allows developers to tap functionality at the right time:

• Hooks before wp load custom features early
• wp hook integrates with core WordPress
• the_post sets up post for theme usage
• wp_head adds JS/CSS assets and meta tags
• wp_footer good for closing tags/adding scripts

Using priority and accepted arguments

Developers can control hook execution using priority and accepted arguments:

Priority

• Defines order of execution when multiple functions hooked to same action
• Default is 10, lower number executes first
• Useful when order of execution matters

Accepted arguments

• Specifies number of parameters passed to function when hook triggers
• Set to 1 for filters so function modifies value passed
• Allows accessing relevant data when hook executes
• Keep required arguments to minimum

Example usage:

// Execute display_count first 
add_action('wp_head', 'display_count', 5); 

// Hook named myplugin_format expects 2 arguments 
add_filter('the_content', 'myplugin_format', 10, 2);

Examples of key WordPress hooks and their usage

Some commonly used hooks include:

wp_head

• Fires inside head tag in header.php
• Add JS, CSS or meta tags
• Usage: Add Google Analytics code

add_action('wp_head', 'insert_ga_script');

function insert_ga_script() {

  // Output GA Embed code  

}

the_content

• Filters main post and page content
• Modify $content passed to function
• Usage: Format content, add sharing buttons

add_filter('the_content', 'modify_content');

function modify_content($content) {

  // Append sharing buttons HTML 
  
  return $content;
}  

save_post

• Fires whenever post or page is updated
• Triggered on both publish and update
• Usage: Perform post-save custom actions

add_action('save_post', 'save_postdata');

function save_postdata($post_id) {

  // Save custom field value
  
}

Troubleshooting hook execution order

Some tips for debugging issues with hook load order:

• Enable WP_DEBUG in wp-config.php
• Install Debug Bar plugin to identify hooked functions
• Check priority parameter for hooks
• Log messages inside hooked functions to isolate order
• Hook test function to wp action to determine theme/plugin load order
• Disable plugins and switch to default theme to isolate issues

Proper use of hooks is critical for extending WordPress functionality. Understanding the hierarchy, action vs filter usage, arguments, and priority assist in tapping WordPress execution flow effectively.