Create custom column in post table in wordpress

You should not be editing core files, it makes it very difficult to upgrade.

Use the add_meta_box() function and put it in a plugin file.

See example below, this plugin will add a meta box to all pages in your wordpress installation. Just edit it for your own means…

<?php 
/*
Plugin Name: META Data Plugin
Plugin URI: http://eddsmith.me
Description: Adds a META description box to all pages.
Version: 1.0
Author: Edd Smith
Author URI: http://eddsmith.me
*/
?>
<?php

add_action('admin_init','metadescription_meta_box_init');

function metadescription_meta_box_init() {

    // The Function that creates the meta box
    add_meta_box('metadescription-meta',__('META description','metadescription-plugin'), 'metadescription_meta_box', 'page','advanced','default'); 

    // hook to save our meta box data when the page is saved
    add_action('save_post','metadescription_save_meta_box');  

}


function metadescription_meta_box($post,$box) {

    // retrieve our custom meta box values
    $metadescription = get_post_meta($post->ID,'_metadescription',true);

    // custom meta box form elements
    echo '<p>' .__('Enter META description below. Remember... <br/>#1 -Search engines truncate the description at 160 characters<br/>#2 - Leave blank and search engines will choose text for you<br/>#3 - Since 2009 META descriptions do not influence Googles ranking algorithms. ','metadescription-plugin'). ': <input type="text" name="metadescription" value="'.esc_attr($metadescription).'" style="width:100%;"></p>';    

}


function metadescription_save_meta_box($post_id,$post) {
    // if post is a revision skip saving our meta box data
    if($post->post_type == 'revision') { return; }


    // process form data if $_POST is set
    if(isset($_POST['metadescription'])) {

    // save the meta box data as post meta using the post ID as a unique prefix
    update_post_meta($post_id,'_metadescription', esc_attr($_POST['metadescription']));
    }
}


?>