Wordpress Hooks

Something that often confuses developers when starting to work with WordPress are the use of apply_filters() and add_action(). You will often see plugins/themes making use of these in code and if you don't understand the concept, you will find it hard to work with them.

In brief (very brief, look up WordPress load flowchart for process in detail), WordPress loads in the following way:

1. wp-load.php - functions etc
2. mu-plugins - any files found in the mu-plugins folder - often used to serve cached objects
3. Plugins - no particular order, any installed and activated plugins will be loaded
4. Active child theme / parent theme
5. init - rest of data
6. template

If you are a developer and working with a functions file, you can see both are loaded earlier in the process than the files you are working with. Meaning you can't modify processes (note you cannot overwrite functions) or variables that run later or have not been defined yet. Also theme developers may place hooks in their code to allow plugins to hook to or plugins might allow for other plugins to overwrite their variables. Now this may be confusing thus far, but hang in there.

To understand add_filter() and add_action() we need to look at how the hooks are created in the first place.

$arga= 'hello';
do_action('im_a_hook', $arga );

When you encounter the above in WordPress, it will call any functions attached to the hook im_a_hook (look up $wp_filter for information on the process). In your attached function $arga will be available for the attached function to work with.

add_action('im_a_hook', 'attached_function');

function attached_function($arga){
     echo $arga;
}

This opens up powerful new opportunities to modify variables at certain points of the load process. Remember we said earlier that templates are loaded after plugins/ themes? One common plugin is WooCommerce which creates screens later in the process, I'm not going to document how but an example of do_action can be found in the plugin.

do_action( 'woocommerce_after_add_to_cart_button' );

Here we have a hook created that passes no variables back, but we can still have fun with it:

add_action( 'woocommerce_after_add_to_cart_button', 'special_offer');

function special_offer(){
    echo '<h1>Special Offer!</h1>;
}

The above add_action will echo a heading of special offer where do_action('woocommerce_after_add_to_cart_button') is located which is when creating a WooCommerce screen. So we can use this hook to insert html. Other uses might include redirecting to a different screen altogether, etc.

Also multiple variables may be passed to the function. Try this in your themes functions. Note the last parameter we are setting to 3, because we want to work with the 3 available parameters. If we changed this to 2, only 2 would be returned and we would get a undefined error.

add_action('custom_hook', 'attached_function', 10, 3);

function attached_function($a,$b,$c){
    
    var_dump($a);
    var_dump($b);
    var_dump($c);
    
}
    

$arga = 1;
$argb = 2;
$argc = 3;

do_action('custom_hook', $arga, $argb, $argc);
exit;

There is another WP hook type called a filter. A filter is different from an action in its usage, an action can only receive variables, obviously these variables are within the functions scope (you should know what php scope is, if not google). Filters pass back the returned data, so you can use to modify variables.

$filter_me= apply_filters('im_a_filter', $variable_to_filter);

Where you see the above, you can modify the value of $filter_me as any data you return will be the value stored in the variable. So for example (note we are changing $variable_to_filter to $filter_me in the example):

add_filter('im_a_filter', 'attached_function', 100);

function attached_function($filter_me){
    
    $filter_me= 'ray';
    
    return $filter_me;
    
}


$filter_me = 'bob';
$filter_me= apply_filters('im_a_filter', $filter_me);

The $filter_me variable will now contain 'ray' rather than 'bob', we have set a priority of 100 so we are reasonably confident no one is changing the value after use (there can be multiple filters running on the same hook) So we can now change variables used later in the process if apply_filters() is present.

You can also pass multiple parameters, but you can only change the value of one. You must also return a value, or else your variable will contain nothing. If you understand how you use php to assign values/arrays/objects to variables this will be obvious to you, e.g.:

add_filter('im_a_filter', 'attached_function', 100, 3);

function attached_function($filter_me, $arga, $argb){
    
    $filter_me= 'ray'.$arga.$argb;

    $arga= 'you fool';
    
    return $filter_me;
    
}

$filter_me = 'bob';

$arga = ' middlename';
$argb = ' surname';

$filter_me= apply_filters('im_a_filter', $filter_me, $arga, $argb);

The $filter_me variable now contains 'ray middlename surname'. But what about $arga? This still contains 'middlename', changing an $arga to 'you fool' within our function has no effect on the defined value outside of its scope (there are ways, google globals etc.)

add_action($hook_name, $function, $priority, $parameters)

add_filter($hook_name, $function, $priority, $parameters);