How to properly turn off REVISIONS and AUTOSAVE for whole site and optionally for a custom post type only

This should be placed in your wp-config.php (and no where else):

define( 'AUTOSAVE_INTERVAL', 60*60*60*24*365 ); // Set autosave interval to 1x per year
define( 'EMPTY_TRASH_DAYS',  0 ); // Empty trash now: Zero days
define( 'WP_POST_REVISIONS', false ); // Do not save andy revisions

Placing the defines in wp-config.php is fine until you turn WP_DEBUG on when you will get 'already defined' PHP notices in the debug.log every couple of minutes. Others claim placing these defines above the ABSPATH define will help.

However, I can categorically confirm the best place to put your defines is in a plugin, because the activated plugins are loaded before the WP default defines.

The default defines are protected with if exists tests, hence your plugin loaded defines will take precedence and will not cause a clash nor the repetitive PHP notices in the debug log.

I'm also looking for how to disable autosave. But here's what I was told in a Trac ticket:

If you really need this feature you should manage the sequential IDs yourself in a custom field and then implement custom URL routing. It shouldn't be too hard to pull that off.