WordPress
Meta Box

Introducción

Sintaxis

• _x ('Text', 'Description', 'textdomain') se usa para agregar una descripción para el servicio de traducción en lugar de __ ('Text', 'textdomain') que es solo la traducción
• _ex ('Text', 'Description', 'textdomain') se usa para hacer eco del texto traducido con una descripción

Observaciones

Un ejemplo simple con una entrada regular y una entrada de selección

/**
* Add meta box to post types.
*
* @since  1.0.0
*/
function myplugin_add_meta_box() {
    // Set up the default post types/
    $types = array(
        'post',
    );

    // Optional filter for adding the meta box to more types. Uncomment to use.
    // $types = apply_filters( 'myplugin_meta_box_types', $types );

    // Add the meta box to the page
    add_meta_box(
        'myplugin-meta-box', // Meta Box Id. Can be anything.
        _x( 'Custom Meta', 'Custom Meta Box', 'myplugin' ), // The title of the meta box. Translation is optional. Can just be string.
        'myplugin_render_meta_box', // The render meta box function.
        $types, // Add the post types to which to add the meta box.
        'side', // Show on the side of edit.
        'high' // Show at top of edit.
    );
}

add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );

/**
* Render the meta box.
*
* This shows examples of a basic input and a select inside a meta box. These can be anything.
*
* @since  1.0.0
*
* @param $post WP_Post The post being edited.
*/
function myplugin_render_meta_box( $post ) {
    // Get the current post meta values for our custom meta box.
    $city  = get_post_meta( $post->ID, 'city', true ); // True is for returning a single value.
    $country = get_post_meta( $post->ID, 'country', true ); // True is for returning a single value.

    // Add the WP Nonce field for security.
    wp_nonce_field( plugin_basename( __FILE__ ), 'myplugin_meta_nonce' );
?>

<p>
    <label for="city">
        <?php _ex( 'City', 'Custom Meta Box Template', 'myplugin' ); ?>
    </label>
    <input name="city" id="city" value="<?php echo $city; ?>" />
</p>
<p>
    <label for="country">
        <?php _ex( 'Country', 'Custom Meta Box Template', 'myplugin' ); ?>
    </label>
    <select name="country" id="country">
        <option value="United States" <?php selected( $country, 'United States' ); ?>><?php _ex( 'United States', 'Custom Meta Box Template', 'myplugin' ); ?></option>
        <option value="Mexico" <?php selected( $country, 'Mexico' ); ?>><?php _ex( 'Mexico', 'Custom Meta Box Template', 'myplugin' ); ?></option>
        <option value="Canada" <?php selected( $country, 'Canada' ); ?>><?php _ex( 'Canada', 'Custom Meta Box Template', 'myplugin' ); ?></option>
    </select>
</p>

<?php
}

/**
* Save meta box data.
*
* @since  1.0.0
*
* @param $post_id int The Id of the Post being saved.
*/
function myplugin_save_meta_data( $post_id ) {
    // Verify this is not an auto save.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Validate the meta box nonce for security.
    if ( ! isset( $_POST['myplugin_meta_nonce'] ) || ! wp_verify_nonce( $_POST['myplugin_meta_nonce'], plugin_basename( __FILE__ ) ) ) {
        return;
    }

    // Get the new values from the form.
    $city    = $_POST['city'];
    $country = $_POST['country'];

    // update_post_meta will add the value if it doesn't exist or update it if it does.
    update_post_meta( $post_id, 'city', $city );
    update_post_meta( $post_id, 'country', $country );

    /*
    * OPTIONAL ALTERNATIVE
    *
    * Instead of just using update_post_meta, you could also check the values and 
    * issue create/update/delete on the post meta value.
    */
    // $current_city_value = get_post_meta( $post_id, 'city', true ); // True is for returning a single value.
    //
    // // Add the post meta if it doesn't exist.
    // if ( $city && '' === $city ) {
    //     add_post_meta( $post_id, 'city', $city, true ); // True means the key is unique to the post. False is default and more than one can be added.
    // }
    // // Edit the post meta if it does exist and there is a new value. 
    // elseif ( $city && $city != $current_city_value ) {
    //     update_post_meta( $post_id, 'city', $city );
    // } 
    // // Delete the post meta if there is no new value. 
    // elseif ( '' === $city && $current_city_value ) {
    //     delete_post_meta( $post_id, 'city', $current_city_value ); // $current_city_value is optional and is used to differentiate between other values with the same key.
    // }
}

add_action( 'save_post', 'myplugin_save_meta_data' );