cookie_sidebar.php

<?php
/*
Plugin Name: WP Shortcode Example
Description: It's a shortcode!
Version: 0.1.0
Author: Sally
*/

function cookie_sidebar() {
	ob_start();
	?>
	<div id="cookie_sidebar">
		Loading...
	</div>

	<button id="setCookieButton">Set Cookie</button>

	<!-- TODO: This JavaScript should be in a seperate file. -->
	<script type="text/javascript">
	var ajax_url = "<?= admin_url( 'admin-ajax.php' ); ?>";
	jQuery.post(ajax_url, { 'action': 'cookie_sidebar_pick' }, function(response) {
		jQuery('#cookie_sidebar').html(response);
	});

	jQuery('#setCookieButton').on('click', function() {
		createCookie('preferredMember', 'true');
		alert('cookie set!');
	});


	function createCookie(name, value, days) {
		var expires = "";
		if (days) {
			var date = new Date();
			date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
			expires = "; expires=" + date.toUTCString();
		}
		document.cookie = name + "=" + value + expires + "; path=/";
	}
	</script>
	<?php
	return ob_get_clean();
}

add_shortcode( 'cookie_sidebar', 'cookie_sidebar' );

function cookie_sidebar_pick() {
	// TODO: This HTML should be pulled from another file.
	$sidebar1 = <<<EOT
<div id="sidebar1" class="hide">
	Welcome preferred member!
	<img src="https://unsplash.it/600/200.png">
</div>
EOT;

	$sidebar2 = <<<EOT
<div id="sidebar2" class="hide">
	Welcome, please consider signing up!
	<img src="https://unsplash.it/600/210.png">
</div>
EOT;

	if ( $_COOKIE['preferredMember'] === 'true' ) {
		echo $sidebar1;
	} else {
		echo $sidebar2;
	}
	wp_die();
}

add_action( 'wp_ajax_nopriv_cookie_sidebar_pick', 'cookie_sidebar_pick' );
add_action( 'wp_ajax_cookie_sidebar_pick', 'cookie_sidebar_pick' );