How to Use add_filter in WordPress: Simple Guide

Use add_filter in WordPress to modify data by hooking your custom function to a filter hook. It takes the filter name, your callback function, priority, and accepted arguments to change output before it is used.

Syntax

The add_filter function connects your custom function to a WordPress filter hook. It has four parts:

• filter_name: The name of the filter hook you want to modify.
• callback_function: Your function that changes the data.
• priority (optional): Order to run your function (default is 10).
• accepted_args (optional): Number of arguments your function accepts (default is 1).

php

add_filter( string $filter_name, callable $callback_function, int $priority = 10, int $accepted_args = 1 );

Example

This example changes the WordPress site title by hooking a function to the bloginfo filter. It adds " - Modified" to the title.

php

<?php
function modify_site_title($title) {
    return $title . ' - Modified';
}
add_filter('bloginfo', 'modify_site_title', 10, 2);
?>

Output

If the site title was "My Blog", it will display as "My Blog - Modified" wherever bloginfo('name') is used.

Common Pitfalls

Common mistakes when using add_filter include:

• Not matching the number of arguments your callback accepts with accepted_args.
• Using the wrong filter hook name.
• Forgetting to return the modified value in your callback function.
• Setting priority incorrectly, causing your filter to run too early or late.

php

<?php
// Wrong: callback does not return value
function wrong_filter($content) {
    $content .= ' Extra text';
    // Missing return statement
}
add_filter('the_content', 'wrong_filter');

// Correct: callback returns modified value
function correct_filter($content) {
    $content .= ' Extra text';
    return $content;
}
add_filter('the_content', 'correct_filter');
?>

Quick Reference

Remember these tips when using add_filter:

• Always return the modified value in your callback.
• Check the filter hook documentation for how many arguments it passes.
• Use priority to control the order if multiple filters modify the same data.
• Test your filter to ensure it works as expected.

Key Takeaways

Use add_filter to hook your function to a WordPress filter and modify data.

Your callback function must return the modified value to work correctly.

Match the accepted_args parameter to the number of arguments your function uses.

Use priority to control when your filter runs relative to others.

Always check the filter hook documentation for correct usage.