nocache_headers() │ WP 2.0.0

Cancel page caching before executing any code related to sessions or that outputs information that should always be up-to-date:

// set the headers overriding the browser cache
nocache_headers();

// execute the code

Suppose we need our AJAX request cached in the browser and when this request is repeated for a specified period of time, the browser takes data from the cache and does not make an AJAX request.

This can be accomplished by setting the right headers for caching. This can be done either directly or via the [nocache_headers] filter (/hook/nocache_headers).

Option: through the nocache_headers filter

You need to insert the filter directly into the PHP code and check in the filter that it works for our query, so that caching works for other queries.

Suppose our AJAX event is called my_ajax_action, then:

add_filter( 'nocache_headers', function( $headers ){

	// enable caching only for our event
	if( $_REQUEST['action'] === 'my_ajax_action' ){
		$life_time = HOUR_IN_SECONDS; // cache for one hour

		$headers = array(
			'Expires'       => gmdate( 'D, d M Y H:i:s', time() + $life_time ) .' GMT',
			'Cache-Control' => 'public',
		);
	}

	return $headers;
});

// AJAX request handler function
add_action( 'wp_ajax_'.'my_ajax_action', 'my_ajax_action_function' );
function my_ajax_action_function(){
	// do something and return the result to the AJAX request
	// the result of this request will be cached in the browser!
}

The upside of this option:
You can set caching for several events at once by simply adding a name check to the event.

Option: tense in the code

Directly in the code is often more convenient, because the headers can be set directly in the AJAX request handler function.

Suppose our AJAX event is called my_ajax_action, then:

// AJAX request handler function
add_action( 'wp_ajax_'.'my_ajax_action', 'my_ajax_action_function' );
function my_ajax_action_function(){

	// enable caching in the browser
	$life_time = HOUR_IN_SECONDS; // cache for one hour
	header( 'Expires: ' . gmdate('D, d M Y H:i:s', time() + $life_time) . ' GMT' );
	header( 'Cache-control: public' );

	// do something and return the result for the AJAX request
	// the result of this request will be cached in the browser!
}

The upside of this option:
Less code and setting caching can be done directly in the handler function.

Let's add custom headers for all calls of wp_get_nocache_headers() function. It calls, for example, for all admin pages. We use nocache_headers hook to do this work:

add_filter( 'nocache_headers', function( $headers ) {

	return [
		'Cache-Control' => 'no-store, no-cache, must-revalidate, max-age=0, some-custom-thing',
		'Pragma'        => 'no-cache',
		'Expires'       => gmdate( 'D, d M Y H:i:s \G\M\T', time() )
	] + $headers;
} );