Please explain me what the do_action does

Question

I am trying hard to understand do_action but not understanding. I was studying the blane theme. And they have this code in sidebar.php

<div id="secondary" class="widget-area col-md-3" role="complementary">
        <?php do_action( 'before_sidebar' ); ?>
        <?php if ( ! dynamic_sidebar( 'sidebar-1' ) ) : ?>

            <aside id="search" class="widget widget_search">
                <?php get_search_form(); ?>
            </aside>

            <aside id="archives" class="widget">
                <h1 class="widget-title"><?php _e( 'Archives', 'blain' ); ?></h1>
                <ul>
                    <?php wp_get_archives( array( 'type' => 'monthly' ) ); ?>
                </ul>
            </aside>

            <aside id="meta" class="widget">
                <h1 class="widget-title"><?php _e( 'Meta', 'blain' ); ?></h1>
                <ul>
                    <?php wp_register(); ?>
                    <li><?php wp_loginout(); ?></li>
                    <?php wp_meta(); ?>
                </ul>
            </aside>            

        <?php endif; // end sidebar widget area ?>

    </div><!-- #secondary --> 

But i don't get what is the second line i.e <?php do_action( 'before_sidebar' ); ?> doing. When i remove this line still it outputs the same result.

Can anybody please explain with some example.

Answer 1

If you hook some functions to the before_sidebar action, they will be executed in your code. Your action is now probably without any function hooked, so it returns nothing.

Example:

<?php
add_action( 'before_sidebar', function() {
  echo 'Try me!';
});
add_action( 'before_sidebar', function() {
  echo 'Yep. ';
}, 1);

// this should output "Yep. Try me!"
<?php do_action( 'before_sidebar' ); ?>

The third argument is a priority. In my example I've set the priority to 1 to the second function and it will be executed at first place.

For more information take a look at WP Codex do_action, and add_action.