WordPress Flash Messaging

wp-flash.php

<?php

/**
 * Triggers validation when a post is saved.
 */
add_action(
	'save_post',
	'validate_post',
	10,
	3
);

/**
 * Post validation.
 */
function validate_post( $id = null, $post = null, $update ) {
	if ( wp_is_post_autosave( $post_ID ) ) {
		return;
	}

	// Do not validate new posts.
	/*
	if ( ! $update ) {
		return;
	}
	*/

	// Prevent multiple executions.
	if ( did_action( 'save_post' ) > 1 ) {
		return;
	}

	// Validation logic goes here.
	set_message( 'This is a vendor: ' . microtime( true ) );
}

/**
 * Triggeras the display of admin notices.
 */
add_action(
	'admin_notices',
	function () {
		check_messages();
	}
);

/**
 * Checks for admin notices.
 */
function check_messages() {
	$msgs = json_decode( get_messages(), true );
	if ( $msgs ) {
		foreach ( $msgs as $msg ) {
			?>
		<div class="notice notice-success is-dismissible">
			<p><?php echo $msg; ?></p>
		</div>
			<?php
		}
	}
	delete_transient( 'my-custom-notices' );
}

/**
 * Gets admin notices.
 */
function get_messages() {
	return get_transient( 'my-custom-notices' );
}

/**
 * Sets admin notices.
 */
function set_message( $msg ) {
	$msgs   = json_decode( get_messages(), true );
	$msgs[] = $msg;
	set_transient( 'my-custom-notices', json_encode( $msgs ) );
}