Why callback function is not working in wp_head hook?

Question

I have two versions of the simplest function that hooked to the wp_head action.

One is working, andother one is not.

Can someone explain?

// Works !
function custom_description() {
     ?>
         <meta name="description" content="cococo" />
     <?php  
}

// Does not work
/*
function custom_description() {   
    return  '<meta name="description" content="cococo" />';
}
*/

add_action( 'wp_head', 'custom_description' );

Answer 1

I found answer i do not need to return value, i must echo it. So my function should look like:

function custom_description(){   
    echo  '<meta name="description" content="cococo" />';
 }

This works !

Answer 2

You can easily recognize where you're supposed to return a value, and where you're supposed to do an action (like echo or some other action).

Although, internally they are very similar, but:

1. Any hook that's called a filter hook, and used with add_filter function, expects a return.

2. On the other hand, any hook that's called an action hook, and used with add_action function, doesn't expect a return and instead expects an action (like echo or something else).

---

Now, in your code, since wp_head is an action hook:

add_action( 'wp_head', 'custom_description' );

you're supposed to do an action like echo, instead of return.

---

You'll get more info. from the following documents:

1. https://developer.wordpress.org/plugins/hooks/#actions-vs-filters

2. https://developer.wordpress.org/plugins/hooks/actions/

3. https://developer.wordpress.org/plugins/hooks/filters/