How-to inspect filter-able $vars?

Note: This is more of a tutorial/wiki than a real question and should be a reference for later Questions. If you got something to add, please feel free to add an answer. Working answers get upvoted. 🙂

Scenario

You want to modify the output of some wp core function and instead of modifying the core directly (which is always bad), you have found a call like this:

$filterable_var = apply_filters( 'name_of_filter', $filterable_var );

The filter function allows you to modify eg. $filterable_var and therefore the output of the wp function.

The Problem

You don’t know what exactly the function wants as input.

The Tags

Check what you can modify.

Possibly Related Q:
[Fell free to add your own Questions here.]

• How-to inspect global variables?

Answers:

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Method 1

Example

Nav menu walker – allows adding eg. css classes to (all) menu items.

// copyied from /wp-core/wp-includes/nav-menu-template.php > line 76 (wp 3.1.1) - start_el() function
$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );

Now let’s check the $var:

function wpse15319_check_nav_menu_classes( $classes ) 
{
    // You can take any of the $vars from the core function above: $classes, $item, $args
    echo '<pre>';
        // nice list:
        print_r( $classes ); 
        // or check of what type the $var is
        var_dump( $classes );
    echo '</pre>';

    return $classes;
}
add_filter( 'nav_menu_css_class', 'wpse15319_check_nav_menu_classes', 10 );

Modify – Applied example

function wpse15319_add_nav_menu_classes( $classes ) 
{
    $classes[] = '';
    $classes[] .= 'my added css classes'; 

    return $classes;
}
add_filter( 'nav_menu_css_class', 'wpse15319_add_nav_menu_classes', 10 );

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0