plugins - What exactly happens to function argument availability when using a filter?

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The goal of this question is:

• to be able to interpret WP docs better
• to understand what's exactly happening in this scenario
• to verify what I'm thinking is correct

---

I came across this code.

    function register() {
        ...
        add_filter( "plugin_action_links_$this->plugin", array( $this, 'settings_link' ) );
    }

    public function settings_link($links) {
        $settings_link = '<a href="admin.php?page=alecaddd_plugin">Settings</a>';
        array_push( $links, $settings_link );
        return $links;
    }

When I debug settings_link($links), I see $links is an array with a 'deactivate' key and value.

Looking at wp dev docs, it seems $links relates to the $actions parameter.
apply_filters( "plugin_action_links_{$plugin_file}", string[] $actions, string $plugin_file, array $plugin_data, string $context )

I noticed though, that the other parameters, $plugin_file, $plugin_data, $context, were not available in the debugger.

I then changed the code as follows.

        function register() {
            ...
            add_filter( "plugin_action_links_$this->plugin", array( $this, 'settings_link' ), 10, 4 ); );  // change 1
        }

        public function settings_link($links, $plugin_file, $plugin_data, $context ) {  // change 2
        $settings_link = '<a href="admin.php?page=alecaddd_plugin">Settings</a>';
            array_push( $links, $settings_link );
            return $links;
        }

Low and behold, now the other parameters $plugin_file, $plugin_data, $context are now available in my function.

I suspect calling add_filter while only passing a callback, is some form of workflow to make things faster & easier.

Can somebody clarify further, or just confirming, what's exactly happening here?
Is this behavior documented somewhere in the official docs?

The goal of this question is:

• to be able to interpret WP docs better
• to understand what's exactly happening in this scenario
• to verify what I'm thinking is correct

---

I came across this code.

    function register() {
        ...
        add_filter( "plugin_action_links_$this->plugin", array( $this, 'settings_link' ) );
    }

    public function settings_link($links) {
        $settings_link = '<a href="admin.php?page=alecaddd_plugin">Settings</a>';
        array_push( $links, $settings_link );
        return $links;
    }

When I debug settings_link($links), I see $links is an array with a 'deactivate' key and value.

Looking at wp dev docs, it seems $links relates to the $actions parameter.
apply_filters( "plugin_action_links_{$plugin_file}", string[] $actions, string $plugin_file, array $plugin_data, string $context )

I noticed though, that the other parameters, $plugin_file, $plugin_data, $context, were not available in the debugger.

I then changed the code as follows.

        function register() {
            ...
            add_filter( "plugin_action_links_$this->plugin", array( $this, 'settings_link' ), 10, 4 ); );  // change 1
        }

        public function settings_link($links, $plugin_file, $plugin_data, $context ) {  // change 2
        $settings_link = '<a href="admin.php?page=alecaddd_plugin">Settings</a>';
            array_push( $links, $settings_link );
            return $links;
        }

Low and behold, now the other parameters $plugin_file, $plugin_data, $context are now available in my function.

I suspect calling add_filter while only passing a callback, is some form of workflow to make things faster & easier.

Can somebody clarify further, or just confirming, what's exactly happening here?
Is this behavior documented somewhere in the official docs?

• plugins
• filters
• hooks

Share Improve this question edited Oct 1, 2021 at 11:18 Jacob Peattie 44.1k1010 gold badges5050 silver badges6464 bronze badges asked Oct 1, 2021 at 11:11 progonkpaprogonkpa 10333 bronze badges 2

• 1 I'm not sure I fully understand. Before you had add_filter('..', [..], 10, 1) (because 10 and 1 are the defaults), now you have add_filter('..', [..], 10, 4). With the 4 you instruct WP to pass 4 arguments to this method. And yes, add_filter and add_action basically allow you to change somebody else's behaviour. Otherwise you'd have to override their code, which would get undone by an update. – kero Commented Oct 1, 2021 at 11:42
• 1 @kero that's the missing piece of information I needed. I didn't know the default value for the number of arguments was 1. That explains everything. I could have known from the add_filter docs.. anyway, thank you. – progonkpa Commented Oct 1, 2021 at 11:48

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1 Answer 1

Sorted by: Reset to default 2

The important thing to note is that action and filter hooks are mostly just standard PHP callback behaviour.

The second argument passed to add_filter() should be a PHP Callable. When the filter is applied with apply_filters(), that callback is just run through the core PHP function call_user_func_array(). Any additional arguments to apply_filters() are then passed as $args to your callback.

The WordPress specific behaviour is around defining the number of arguments your callback accepts. When you add a filter with add_filter(), the 4th argument defines the number of arguments that the callback accepts. WordPress then only passes that number of arguments when it calls call_user_func_array().

Normally passing more arguments than a callback accepts is perfectly fine. The exception is built-in PHP functions, which will throw an error. For example, this will throw an error:

call_user_func_array( 'addslashes', array( 'argument one', 'argument two' ) );

To prevent this WordPress makes you declare how many arguments your callback accepts, and trims the number passed to call_user_func_array() accordingly.

There is an old ticket about deprecating this behaviour here, and you can follow the discussion to see why that never happened. The short version is performance and backwards compatibility.

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