Using add_action hook in WordPress

WordPress hooks change how the WordPress behaves without needing to edit any PHP source files.

There are two types of hooks:

• Actions
• Filters

To use either you need:

• to write a custom function known as a Callback
• register the Callback to a specific action or filter.

In here I will explain actions. This uses add_action hook.

Actions will run at a specific point in time either from the:

• WordPress Core
• WordPress plugins,
• or WordPress themes.

Here I will provide simple working example for the add_action hook in WordPress:

function hello_header() {
  echo "I'm in the header!";
}
add_action('wp_head', 'hello_header');

The result of adding this code to the functions.php file of my theme would be echoing “I’m in the header!” string.

Here wp_head represents the moment before the “head” section for the page is ready. Some other time moments are:

• init — moment when creating a page, both front end and administration
• admin_init — the first thing done on every administration page
• admin_head — the last moment in the administration page’s “head” section
• admin_menu — constructing the navigation menu in the administration pages
• template_redirect — occurs before the theme template file has been chosen
• wp_enqueue_scripts — moment to eliminate duplicate scripts
• wp_enqueue_style — moment to eliminate duplicate styles
• widgets_init — moment for constructing the list of active widgets
• loop_start and loop_end — surrounding the Loop
• wp_footer — moment to insert additional stuff before closing the body tag

And here is the complete WordPress hooks list. And here is the add_action hooks list.

How about adding meta keywords to our post via a hook? How do we get meta keywords from the post tags and categories?

Here is the code I used:

function add_metatags()
{
 if (is_single()){
 // posttags takes all post tags
 $posttags = get_the_tags();
    if ($posttags){
       foreach($posttags as $tag){
          $keywords[] = strtolower($tag->name);
       }
    }
 
 // now adding post categories
 foreach((get_the_category()) as $category){
    $keywords[] = strtolower($category->cat_name);
 }
 $list = implode(", ", array_unique($keywords));
 echo '';
 }
}
 
add_action('wp_head', 'add_metatags');
 

Note, apart from the add_action hooks there are also add_filter hooks described in the tutorial to use add_filter hook.

Filters give you the ability to change data. Callback functions for filters will typically accept a variable, modify it, and than return it.

Filters should never affect the global variables and the output.