Gestión de Meta Box básica en posts

u87.php

add_action( 'add_meta_boxes', 'post_options' );
add_action( 'save_post', 'post_options_save_data' );

function post_options() {

    add_meta_box('post_meta_box_mobile', 'Título de la sección', 'post_meta_box_mobile', 'post', 'normal', 'high');
}

function post_meta_box_mobile($post) {

    $id = $post->ID;
    $availability = get_post_meta($id,"_mobile_availability",true);

    ?>
    <table class="form-table">
        <tbody>
        <tr>
            <th><label for="availability">Availability</label></th>
            <td><input type="text" id="availability" name="availability" value="<?php echo $availability; ?>" /></td>
        </tr>
        </tbody>
    </table>

<?php
}

function post_options_save_data($post_id) {

    if (isset($_POST['availability'])) {
        update_post_meta($post_id, '_mobile_availability', $_POST['availability']);
    }
}