I have add this code to functions.php but startwordpress_scripts() function didn't run.

function startwordpress_scripts() {
    wp_enqueue_style( 'bootstrap', get_template_directory_uri() . '/css/bootstrap.min.css', array(), '3.3.6' );
    wp_enqueue_style( 'blog', get_template_directory_uri() . '/css/blog.css' );
    wp_enqueue_script( 'bootstrap', get_template_directory_uri() . '/js/bootstrap.min.js', array('jquery'), '3.3.6', true );
}

add_action( 'wp_enqueue_scripts', 'startwordpress_scripts' );

So I added following code in header.php and they worked well.

<?php wp_head(); ?>

Now I want to know the magic role of wp_head() function. Thanks.

I have add this code to functions.php but startwordpress_scripts() function didn't run.

function startwordpress_scripts() {
    wp_enqueue_style( 'bootstrap', get_template_directory_uri() . '/css/bootstrap.min.css', array(), '3.3.6' );
    wp_enqueue_style( 'blog', get_template_directory_uri() . '/css/blog.css' );
    wp_enqueue_script( 'bootstrap', get_template_directory_uri() . '/js/bootstrap.min.js', array('jquery'), '3.3.6', true );
}

add_action( 'wp_enqueue_scripts', 'startwordpress_scripts' );

So I added following code in header.php and they worked well.

<?php wp_head(); ?>

Now I want to know the magic role of wp_head() function. Thanks.

• javascript
• css
• wordpress

• What specific question do you have that's not documented? codex.wordpress/Function_Reference/wp_head – Andy Ray Commented Dec 27, 2016 at 3:50
• if you don't add <?php wp_head(); ?> then what ever header.php your theme is using will not be added to the html output for the page. Chances are, most of your other scripts and libraries are linked via in the header.php so if that's not loaded, everything in it is not loaded. – user7234396 Commented Dec 27, 2016 at 3:58

3 Answers 3

In WordPress, actions and filters are considered as hooks. Hooks allow us to modify the WordPress default behaviour without modifying the code found in the core.

Anytime you have an add_action('xxx', 'callback'), the callback function will be called when do_action('xxx') is executed.

In other words: when you have add_action( 'wp_enqueue_scripts', 'startwordpress_scripts' );, it means that WordPress will run startwordpress_scripts() when do_action('wp_enqueue_scripts') is executed.

Now, let's look at the code in the WordPress core.

If you look at its function definition, wp_head() is a "shortcut" to do_action( 'wp_head' );

function wp_head() {
        /**
         * Print scripts or data in the head tag on the front end.
         *
         * @since 1.5.0
         */
        do_action( 'wp_head' );
}

In other words, wp_head() will execute all callbacks that were defined with add_action('wp_head').

If you now look at the wp-includes/default-filters.php file, you'll see:

add_action( 'wp_head', 'wp_enqueue_scripts',1 );

It means that when wp_head() is encountered in your template, a function called wp_enqueue_scripts() is being executed, since it is hooked to wp_head, shown in the line of code above.

the function definition of wp_enqueue_scripts() is:

function wp_enqueue_scripts() {
    /**
     * Fires when scripts and styles are enqueued.
     *
     * @since 2.8.0
     */
    do_action( 'wp_enqueue_scripts' );
}

The do_action( 'wp_enqueue_scripts' ); above is what tells WordPress to execute the callback function of all add_action('wp_enqueue_scripts', 'callback') that are defined.

In short:

1. wp_head() calls do_action('wp_head')
2. do_action('wp_head') executes callback functions of all add_action('wp_head','callback')
3. wp_enqueue_scripts() is one callback of add_action('wp_head','callback')
4. wp_enqueue_scripts() calls do_action('wp_enqueue_scripts')
5. do_action('wp_enqueue_scripts') executes callback functions of all add_action('wp_enqueue_scripts','callback')
6. startwordpress_scripts() is one callback of add_action('wp_enqueue_scripts','callback')
7. Your JS/CSS defined in startwordpress_scripts() are included

The wp_head action hook is triggered within the section of the user's template by the wp_head() function.

If you remove wp_head() function then all action add_action('wp_head','your_custom_action'); is not working also plugin also use wp_head action action to add css or js.

If you remove wp_head function from header.php file then below function not add any JS in header.

function startwordpress_scripts() {
    wp_enqueue_style( 'bootstrap', get_template_directory_uri() . '/css/bootstrap.min.css', array(), '3.3.6' );
    wp_enqueue_style( 'blog', get_template_directory_uri() . '/css/blog.css' );
    wp_enqueue_script( 'bootstrap', get_template_directory_uri() . '/js/bootstrap.min.js', array('jquery'), '3.3.6', true );
}

add_action( 'wp_enqueue_scripts', 'startwordpress_scripts' );

If you want to remove wp_head() function then you need to add like below.

<link rel='stylesheet' id='bootstrap-css' href='http://www.siteurl./wp-content/themes/your-theme/css/bootstrap.min.css?ver=3.3.6' type='text/css' media='all' />
<link rel='stylesheet' id='blog-css' href='http://www.siteurl./wp-content/themes/your-theme/css/blog.css?ver=1.0' type='text/css' media='all' />
<script type='text/javascript' src='http://www.siteurl./wp-content/themes/your-theme/js/bootstrap.min.js?ver=3.3.3'></script>

See Link for wp_enqueue_script function and args

There are no errors in your code shown above and I've double checked to see it working correctly in a fresh WP install. It should not be the cause of your errors.

Take a look at the DEBUG section of the codex to change that. This might need to be dropped into your wp-config.php.

// allow debugging 
defined( 'WP_DEBUG' ) or define( 'WP_DEBUG', true );

// log errors to wp-content/debug.log
defined( 'WP_DEBUG_LOG' ) or define( 'WP_DEBUG_LOG', true );

// show errors on screen
defined( 'WP_DEBUG_DISPLAY' ) or define( 'WP_DEBUG_DISPLAY', true );