Adding A Custom Field Automatically On Post/Page Publish

A code snippet for installing a custom field automatically to a page or post when they are published. You can just add the code below into your functions.php file, located inside your theme’s folder. Of course, don’t forget to change the custom field name.

add_action('publish_page', 'add_custom_field_automatically');
add_action('publish_post', 'add_custom_field_automatically');

function add_custom_field_automatically($post_ID) {
    global $wpdb;
    if(!wp_is_post_revision($post_ID)) {
        add_post_meta($post_ID, 'field-name', 'custom value', true);
    }
}