Difference between do_action and add_action

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This question might be wrong, I am not sure about it. Because I am not clear on this.

I know add_action it is used to to hook our function to the specified function. For example add_action('wp_head'.'myfunc'); now what ever code in myfunc will be executed in the wp_head(). This is clear but i am having doubt in do_action what it does?

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I think it is used to create our own hook like already available hooks(wp_head,wp_footer,..etc) If i am correct can anyone show me a simple understandable answer with simple example.

I have tried the difference in internet but all are pointing to difference between add_action and add_filter. I don’t want to go there because first i want to clarify this and then I’ll move there.

Can anybody help me?

EDIT after Question POST 

function custom_register()
{
    echo '<script>jQuery(document).ready(function(){alert("Learning Hooks");});</script>';

}
do_action('custom');

add_action('custom','custom_register');

I tried this in plugin but i didn’t get the alert message.

But when i hook the same function with wp_head then it is working fine 

/******************working****************/
add_action('wp_head','custom_register');
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4 comments

  1. Use do_action( 'unique_name' ) to create your own actions.

    You can use that to offer an API for your plugin, so other plugins can register callbacks for your custom action. Example: Do I need to call do_action in my plugin?

    But you can use custom actions (or filters) in a theme too. Example: Best practice way to implement custom sections into a WordPress theme

    And you can combine both to make a plugin and a theme working together. Example: How to make method from plugin available in theme?

    Summary: add_action( 'foo' ) registers a callback, do_action( 'foo' ) executes that registered callback.

  2. This is my guess, so if you know better, please make a comment so I can update my guess.

    Your plugin code is executed before wp_head() (which we can assume will invoke the actions added to it). When you add_action('wp_head','custom_register'), you are telling PHP that when (in the future) do_action('wp_head') is called, to call custom_register() as well. The same is true of your call to add_action('custom','custom_register') but as you see in your code, the call to do_action('custom') has already been made, and when it was called, there was not (yet) any action added to it.
    This is why Toscho asked what happens when you call do_action('custom') after you registered the callback. Your answer about back end and front end is ambiguous. If you swap the last two lines in the following code, I think it will work:

    function custom_register()
{
    echo '<script>jQuery(document).ready(function(){alert("Learning Hooks");});</script>';

}
do_action('custom');                    // This is called before it will have an effect.

add_action('custom','custom_register'); // Too late - do_action was already called.

  3. do_action : Registers an action hook while
    add_action : adds a callback function to the registered hook.

    Example

    Consider you wanted to print something before sidebar in you template.

    1. You will add an action hook in your template file index.php via <?php add_action('bp_sidebar_left'); ?>.
    2. Now in your functions.php file you can add a callback function to that hook to print something you want.

    add_action('bp_sidebar_left', 'bp_sidebar_left_cb');
    function bp_sidebar_left_cb() {
    echo 'Hello World !';
    }

  4. You have to use like below:

    function custom_register()
{
    echo '<script>jQuery(document).ready(function(){alert("Learning Hooks");});</script>';

}
.

add_action('custom','custom_register'); // Too late - do_action was already called. 
do_action('custom');                    // This is called before it will have an effect

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