Stopping pop-up ads using PHP involves controlling when and how content is served to users, often by managing sessions or cookies. While PHP cannot directly block client-side pop-up ads created by JavaScript from third-party sources, it can help prevent unwanted popups generated by your own site. Developers can use PHP to set cookies or session variables that track how many times a popup has been shown. For example, if a user has seen a popup once, a PHP script can prevent it from appearing again by checking the stored variable. Additionally, PHP can be used to validate users or filter content to avoid serving ad-heavy pages. Though PHP has limitations on client-side behavior, it supports better control of server-side popup logic.