How to debug save_post actions in WordPress?

I have some custom post meta being generated and am ready to add to a post’s meta. I know how to do this. However, save_post causes a redirection after POST data has been sent. This means I am redirected to the dashboard and lose access to my POST data – therefore I cannot debug easily.

Currently I am using something like:

add_action('save_post', 'something_process');

function something_process() {
   if(defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) 
       return;
   print_r($_POST);
}

Is there a way to easily debug this?

Answer

The best approach for me has been to use a function to log the values to wp-content/debug.log, lifted from http://fuelyourcoding.com/simple-debugging-with-wordpress:

if(!function_exists('log_it')){
 function log_it( $message ) {
   if( WP_DEBUG === true ){
     if( is_array( $message ) || is_object( $message ) ){
       error_log( print_r( $message, true ) );
     } else {
       error_log( $message );
     }
   }
 }
}

Then use the function like this in your save_post hook:

log_it($_POST);
log_it('The value for ' . $custom_field . ' is ' . $_POST[$custom_field]);

Make sure that wp-content/debug.log is writable, and that you have debugging enabled in wp-config.php:

@ini_set('display_errors',0);
define( 'WP_DEBUG',         true );  // Turn debugging ON
define( 'WP_DEBUG_DISPLAY', false ); // Turn forced display OFF
define( 'WP_DEBUG_LOG',     true );  // Turn logging to wp-content/debug.log ON
define( 'WP_POST_REVISIONS', false); // Disables revision functionality