Save an extra custom post field in custom post type in WordPress

rrwd_save_cpf_with_ctp.php

<?php 
/**
* This code saves an extra custom post field with a custom post type to WordPress
* Rian Rietveld - rrwd.nl
* Add this to the functions.php in your WordPress child theme
* 
* Adding the priority 20 for the add_action to save_post is !important, otherwise it won't work 
*/

function rrwd_save_cpf_with_ctp ( $post_id, $post ) {
	// thanks to mathsmith on http://wordpress.stackexchange.com/questions/5688/custom-post-type-edit-publish-hook
	
	// prefix_name_cpf is the name of the custom post field
	// prefix_name_cpt is the name of the custom post type
	
	// Make sure the post obj is present and complete. If not, bail. 
    if(!is_object($post) || !isset($post->post_type)) {
        return;
    }
	
	
	switch($post->post_type) { // Do different things based on the post type

        case "prefix_name_cpt":
            
			if ( is_admin() && ( get_field( 'prefix_name_cpf', $post_id ) ) ) {

				// do not save if this is an auto save routine
				if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return $post_id;
	
				// get the data from the cpf get_field is functionused by the WordPress plugin Advanced Custion Fields by Elliot Condon http://www.advancedcustomfields.com/
				$prefix_name_cpf = get_field( 'prefix_name_cpf', $post_id ); 
				
				// for a regular WordPress call use
				// $prefix_name_cpf = get_post_meta($post_id, 'prefix_name_cpf', true);
	
				// do stuff with this variable
				$prefix_name_extra_cpf = some_function( $prefix_name_cpf );

				// update or add the new cistom post field in the database
				add_post_meta( $post_id, '_prefix_name_extra_cpf', $prefix_name_extra_cpf, true ) 
					or update_post_meta(  $post_id, '_prefix_name_extra_cpf', $prefix_name_extra_cpf );
			}	
            break;

        default:
           	break;

    }
	
	
} // end rrwd_save_cpf_with_ctp

add_action( 'save_post', 'rrwd_save_cpf_with_ctp', 20, 2 );  // priority 20 is !IMPORTANT!


function some_function($extra_var) {
	// do some stuff here
	return $new_var;
	
}

?>