How do I pass arguments for multiple functions hooked to a single action?

Question

I wanted to set a default value for a parameter:

function my_function( $arg = 'foo' ) {
    echo $arg
}
add_action('my_action', 'my_function');

function my_2nd_function( $arg = 'bar' ) {
    echo $arg
}
add_action('my_action', 'my_2nd_function');

But when I call them doing the action

do_action('my_action');

I realized that it didn't print anything. I know I can set up the number of arguments a function can receive

add_action('my_action', 'my_function', 10, 1);

And pass it when doing the action

do_action('my_action', 'foo');

But, what should I do when there are multiple functions hooked on that action? Recommendations?

Answer 1

The value for the default output should be passed to do_action() as the second parameter. E.g.:

do_action( 'my_action', '<p>Default output</p>' );

Note that the above code will not actually output anything. It's just going to fire the callbacks attached to the my_action hook and it will pass '<p>Default output</p>' to those functions.

You can modify the output using callbacks attached to the my_action hook:

add_action( 'my_action', 'my_function', 10, 1 );
function my_function( $default_output ) {
    echo '<p>foo</p>';
}

add_action( 'my_action', 'my_2nd_function', 10, 1 );
function my_2nd_function( $default_output ) {
    echo '<p>bar</p>';
}

add_action( 'my_action', 'my_3rd_function', 10, 1 );
function my_3rd_function( $default_output ) {
    echo $default_output;
}

Output:

<p>foo</p>
<p>bar</p>
<p>Default output</p>

Using a filter might make more sense here. E.g.:

echo apply_filters( 'my_filter', '<p>Default output</p>' );

The output will be determined by the last filter fired on the my_filter hook:

add_action( 'my_filter', 'my_filter_function_1', 10, 1 );
function my_filter_function_1( $default_output ) {
    return $default_output;
}

add_action( 'my_filter', 'my_filter_function_2', 20, 1 );
function my_filter_function_2( $default_output ) {
    return '<p>my_filter_function_2() output</p>';
}

add_action( 'my_filter', 'my_filter_function_3', 30, 1 );
function my_filter_function_3( $default_output ) {
    return '<p>my_filter_function_3() output</p>';
}

Output:

<p>my_filter_function_3() output</p>

If no callbacks are attached to my_filter, the output would be:

<p>Default filter output</p>