gistfile1.php

<?php

// Can be called whenever.
function add_admin_notice( $code, $message, $type = 'error' ) {
	global $wp_admin_notices;

	if ( ! isset( $wp_admin_notices ) )
		$wp_admin_notices = array();

	$wp_admin_notices[] = array(
		'code'    => $code,
		'message' => $message,
		'type'    => $type,
	);
}

// Gets called once, after the 'all_admin_notices' hook.
function do_admin_notices() {
	if ( defined( 'DOING_AJAX') && DOING_AJAX )
		return;

	global $wp_admin_notices;

	if ( ! isset( $wp_admin_notices ) )
		return;

	foreach ( $wp_admin_notices as $notice ) {
		printf( '<div id="setting-error-%1$s" class="%2$s settings-error"><p>%3$s</p></div>',
			$notice['code'],
			$notice['type'],
			$notice['message']
		);
	}

	unset( $GLOBALS['wp_admin_notices'] );
}

// Save all notices on shutdown.
function save_admin_notices() {
	global $wp_admin_notices;

	if ( isset( $wp_admin_notices ) )
		set_transient( 'admin-notices-' . get_current_user_id(), $wp_admin_notices, 30 );
}
add_action( 'shutdown', 'save_admin_notices' );

// Load all notices on init
function load_admin_notices() {
	if ( false !== $notices = get_transient( 'admin-notices-' . get_current_user_id() ) ) {
		global $wp_admin_notices;

		if ( ! isset( $wp_admin_notices ) )
			$wp_admin_notices = array();

		$wp_admin_notices = array_merge( $wp_admin_notices, $notices );
		delete_transient( 'admin-notices-' . get_current_user_id() );
	}

}
add_filter( 'admin_init', 'load_admin_notices' );