Add a meta field to the list of results for a custom post type

There are a couple filters for that. I wrote a tutorial about adding custom columns, which is where the code below is from. The example is adding a column to the ‘page’ post type that displays the page’s template.

You can hook into load-edit.php and use get_current_screen to grab the correct screen ID’s/filters. From there you hook into manage_{$screen->id}_columns and manage_{$screen->post_type}_posts_custom_column

<?php
add_action('load-edit.php', 'pmg_ltt_load');
function pmg_ltt_load()
{
    $screen = get_current_screen();

    if(!isset($screen->post_type) || 'page' != $screen->post_type)
        return;

    add_filter(
        "manage_{$screen->id}_columns",
        'pmg_ltt_add_columns'
    );

    add_action(
        "manage_{$screen->post_type}_posts_custom_column",
        'pmg_ltt_column_cb',
        10,
        2
    );
}

manage_{$screen->id}_columns is what actually adds the column and manage_{$screen->post_type}_posts_custom_column is your callback.

Add the column:

<?php
function pmg_ltt_add_columns($cols)
{
    // pay attention to the key, you'll use it later.
    $cols['template'] = __('Page Template', 'pmg-list-table');
    return $cols;
}

And the callback function will get two argument: the column key (what you used above!) and the post ID. Check to make sure it’s the column you want, then use get_post_meta (or whatever else appropriate to the situation) to fetch the stuff you want to display.

<?php
function pmg_ltt_column_cb($col, $post_id)
{
    static $templates;

    if('template' == $col)
    {
        if(empty($templates))
            $templates = array_flip(get_page_templates());

        $tmp = get_post_meta($post_id, '_wp_page_template', true);
        if($tmp && isset($templates[$tmp]))
        {
            echo esc_html($templates[$tmp]);
        }
        else
        {
            esc_html_e('Default Template', 'pmg-list-table');
        }
    }
}