WordPress: Add new columns to wp-admin posts list

functions.php

<?php

/**
	Add filter and action: Choose one
**/

// Add to all post types
// add_filter('manage_post_posts_columns', 'cg_new_columns_headers');
// add_action('manage_post_posts_custom_column', 'cg_new_columns_content', 10, 2);

// Add to specific post type (replace $posttype with the desired post type slug. eg: post, page, movie, etc)
// add_filter('manage_$posttype_posts_columns', 'cg_new_columns_headers');
// add_action('manage_$posttype_posts_custom_column', 'cg_new_columns_content', 10, 2);

// Add new column header
function cg_new_columns_headers($defaults) {
	$count = 0;
	// Reorder header
	foreach($defaults as $k => $default) {
		if($count == 2) { // Order
			$new_defaults['the_field'] = 'Header';
		}
		$new_defaults[$k] = $default;
		$count++;
	}
	return $new_defaults;
}
 
// The column content
function cg_new_columns_content($column_name, $post_ID) {
    switch ($column_name) :
    	case 'the_field':
    		// example of code
    		$show = get_post_meta($post_ID, 'show_in_slideshow', true);
	    	if($show) echo 'Yes';
	    	break;
	default:
		echo '';
		break;
    endswitch;
}