Passing a parameter to filter and action functions

Question

Is a way to pass my own parameters to the function in add_filter or add_action. For example take a look in the following code:

function my_content($content, $my_param)
{
do something...
using $my_param here ...
return $content;
}
add_filter('the_content', 'my_content', 10, 1);

Can I pass my own parameter? something like:

add_filter('the_content', 'my_content($my_param)', 10, 1)

or

add_filter('the_content', 'my_content', 10, 1, $my_param)

Answer 1

By default this is not possible. There are workarounds if you do it the OOP way.
You could create a class to store the values you want to use later.

Example:

/**
 * Stores a value and calls any existing function with this value.
 */
class WPSE_Filter_Storage
{
    /**
     * Filled by __construct(). Used by __call().
     *
     * @type mixed Any type you need.
     */
    protected $values;

    /**
     * Stores the values for later use.
     *
     * @param  mixed $values
     * @return void
     */
    public function __construct( $values )
    {
        $this->values = $values;
    }

    /**
     * Catches all function calls except __construct().
     *
     * Be aware: Even if the function is called with just one string as an
     * argument it will be sent as an array.
     *
     * @param  string $callback Function name
     * @param  array  $arguments
     * @return mixed
     */
    public function __call( $callback, $arguments )
    {
        if ( is_callable( $callback ) )
        {
            return call_user_func( $callback, $arguments, $this->values );
        }
        // Wrong function called.
        throw new InvalidArgumentException(
            sprintf(
                'File: %1$s<br>
                Line %2$d<br>
                There is no function %3$s',
                __FILE__,
                __LINE__,
                $name
            )
        );
    }
}

Now you can call the class with any function you want – if the function exists somewhere it will be called with your stored parameters.

Let’s create a demo function …

/**
 * Filter function.
 * @param  array $content
 * @param  array $numbers
 * @return string
 */
function wpse_45901_add_numbers( $args, $numbers )
{
    $content = $args[0];
    return $content . '<p>' . implode( ', ', $numbers ) . '</p>';
}

… and use it once …

// First instance of WPSE_Filter_Storage.
$wpse_45901_odd = new WPSE_Filter_Storage(
    array ( 1, 3, 5 )
);
add_filter(
    'the_content',
    // Object instance and called function.
    array ( $wpse_45901_odd, 'wpse_45901_add_numbers' )
);

… and again …

// Second instance of WPSE_Filter_Storage.
$wpse_45901_even = new WPSE_Filter_Storage(
    array ( 2, 4, 6 )
);
add_filter(
    'the_content',
    array ( $wpse_45901_even, 'wpse_45901_add_numbers' )
);

Output:

The key is reusability: You can reuse the class (and in our examples also the function).

PHP 5.3+

If you can use a PHP version 5.3 or newer closures will make that much easier:

$param1 = '<p>This works!</p>';
$param2 = 'This works too!';

add_action(
    'wp_footer',
    function() use ( $param1 )
    {
        echo $param1;
    },
    11
);
add_filter(
    'the_content',
    function( $content ) use ( $param2 )
    {
        return t5_param_test( $content, $param2 );
    },
    12
);

/**
 * Add a string to post content
 *
 * @param  string $content
 * @param  string $string This is $param2 in our example.
 * @return string
 */
function t5_param_test( $content, $string )
{
    return "$content <p><b>$string</b></p>";
}