如何将值添加到当前cookie值

Question

我正在为网站做广告叠加。用户必须在24小时内连续3次看到该广告。首次看到广告后24小时（设置cookie时），循环再次开始

我已经整理了24小时cookie到期但我不知道如何实施价值体系。一旦cookie值达到3，广告就会隐藏

我对php解决方案持开放态度

以下是目前编码的脚本：

function createCookie(name,value,days) {
	if (days) {
		var date = new Date();
		date.setTime(date.getTime()+(days*24*60*60*1000));
		var expires = "; expires="+date.toGMTString();
	}
	else var expires = "";
	document.cookie = name+"="+value+expires+"; path=/";
}

function readCookie(name) {
	var nameEQ = name + "=";
	var ca = document.cookie.split(';');
	for(var i=0;i < ca.length;i++) {
		var c = ca[i];
		while (c.charAt(0)==' ') c = c.substring(1,c.length);
		if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
	}
	return null;
}

function eraseCookie(name) {
	createCookie(name,"",-1);
}

<script type="text/javascript">
var days = 1;
var advert = document.getElementById('overlay-stage');
if (readCookie('seenAdvert')) {
    advert.className = 'htmlNoPages hidden';
} else {
    advert.className = 'htmlNoPages show';
    createCookie('seenAdvert', 'yes', 1);
}
</script>

非常感谢任何帮助