/*
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
 /*
  * Adds a box to the main column on the Post and Page edit screens.
  */
function jma_add_linkedin_input_box()
{
    $screens = array('volunteer');
    foreach ($screens as $screen) {
        add_meta_box(
             'jma_linkedin_input_section',
             __('Linkedin address', 'jma_textdomain'),
             'jma_linkedin_input_box',
             $screen,
             'normal',
             'high'
         );
    }
}
add_action('add_meta_boxes', 'jma_add_linkedin_input_box');
 /*
  * Prints the box content.
  *
  * @param WP_Post $post The object for the current post/page.
  */
function jma_linkedin_input_box($post)
{
    // Add an nonce field so we can check for it later.
    wp_nonce_field('jma_linkedin_input_box', 'jma_linkedin_input_box_nonce');
    /*
     * Use get_post_meta() to retrieve an existing value
     * from the database and use the value for the form.
     */
    $linkedin_values = array();
    if (get_post_meta($post->ID, '_jma_linkedin_data_key', true)) {
        $linkedin_values = get_post_meta($post->ID, '_jma_linkedin_data_key', true);
    }
    $linkedin_html = isset($linkedin_values['linkedin_text'])? $linkedin_values['linkedin_text']:'';
    echo '<p></p>';
    echo '<label for="linkedin_text">';
    _e('linkedin address.', 'jma_textdomain');
    echo '</label><br/>';
    echo '<input name="linkedin_text" size="80" type="text" value="'.$linkedin_html.'"/>';
}

function jma_save_linkedin_postdata($post_id)
{
    /*
     * We need to verify this came from the our screen and with proper authorization,
     * because save_post can be triggered at other times.
     */
    // Check if our nonce is set.
    if (!isset($_POST['jma_linkedin_input_box_nonce'])) {
        return $post_id;
    }
    $nonce = $_POST['jma_linkedin_input_box_nonce'];
    // Verify that the nonce is valid.
    if (!wp_verify_nonce($nonce, 'jma_linkedin_input_box')) {
        return $post_id;
    }
    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return $post_id;
    }
    // Check the user's permissions.
    if ('page' === $_POST['post_type']) {
        if (!current_user_can('edit_page', $post_id)) {
            return $post_id;
        }
    } else {
        if (!current_user_can('edit_post', $post_id)) {
            return $post_id;
        }
    }
    /* OK, its safe for us to save the data now. */
    // Sanitize user input.
    $jma_data['linkedin_text'] = sanitize_text_field($_POST['linkedin_text']);
    // Update the meta field in the database.
    update_post_meta($post_id, '_jma_linkedin_data_key', $jma_data);
}
add_action('save_post', 'jma_save_linkedin_postdata');