WordPress admin custom columns.

wp-custom-columns.php

<?php 
/**
 * @author : Bhaskar K C
 * generic class for admin columns
 * Assuming column values are to be stored in post meta 
 *
 * Columns values can be extended with following usage example
 * add_action( 'twr_admin_columns', 'callback_function_name', 2, 10 );
 * function callback_function_name( $post_type, $column_key ) { // do something here }
 */

if( ! class_exists('TWR_admin_columns') ) {

	class TWR_admin_columns {

		private $post_type;
		private $column_arr = array();
		
		public function __construct( $post_type, $column_array ) {
			
			if( ! is_admin() ) {
				return false;
			}
			
			if( empty( $post_type ) || empty( $column_array ) ) { 
				return true;
			}

			$this->post_type 	= $post_type;
			$this->column_arr 	= $column_array;
			
			// Competition admin custom columns
			add_filter( "manage_{$post_type}_posts_columns", array( &$this, "define_custom_columns" ) );
			add_filter( "manage_edit-{$post_type}_sortable_columns", array( &$this, "sortable_admin_columns" ) );
			add_action( "manage_{$post_type}_posts_custom_column", array( &$this, 'custom_column_value' ) );
		}

		function define_custom_columns( $columns ) {
		    return ( ! empty ( $this->column_arr ) ? $this->column_arr : $columns );
		}

		function sortable_admin_columns( $sortable_columns ) {

			if( empty( $this->column_arr) ) return $sortable_columns;
		    
		    foreach( $this->column_arr as $sortable_column_key => $column_value ) {
		        $custom_sortable_columns[$sortable_column_key] = $sortable_column_key;
		    }

		    return ( ! empty ( $custom_sortable_columns ) ? $custom_sortable_columns : $sortable_columns );

		}

		function custom_column_value( $column ) {

		    if( empty( $this->column_arr) ) return $column;
	        
			// adding action to provide flexibility in terms of calculating column values
			do_action('twr_admin_columns', $this->post_type, $column );

			// Assuming column values are in post meta, so that retrieving them
		    global $post; 
		    $post_id = $post->ID;
		    
		    foreach( $this->column_arr as $column_key => $column_title ) {
			    if( $column != $column_key ) continue;
		        $custom_sortable_columns[$column_key] 	= $column_key;
		        $column_value 				= get_post_meta( $post_id, $column_key, true );
		        echo ( ! empty( $column_value )? $column_value: '' );
		    }
		}
	}
}

// Usage examples.
// Competition admin custom column array
function get_competition_admin_column_arr() {
    return array(
        'cb'            =>'<input type="checkbox" />',
        'title'         =>'Title',
        'entrants'      =>'Entrants',
        'start_date'    =>'Start time',
        'duration'      =>'Duration',
        'status'        =>'Status'
    );
}
$competition_columns = new TWR_admin_columns( 'competition', get_competition_admin_column_arr() );

// Extending custom column value example.
add_action( 'twr_admin_columns', 'custom_admin_column_val', 2, 10 );

function custom_admin_column_val( $post_type, $column_key ) {
    if( $post_type == 'competition' ) {
        switch ( $column_key ) {
            case 'status':
                echo 'Not active';
            break;
        }
    }
}