Facebook: Facebook initially used PHP, and while they have developed and incorporated other technologies over time, PHP was crucial in the early development stages.
Wikipedia: Wikipedia, the world's largest online encyclopedia, is powered by MediaWiki, which is written in PHP.
WordPress: WordPress, one of the most popular content management systems (CMS) for website creation, is built using PHP. Many websites and blogs use WordPress as their platform.
Yahoo: Yahoo uses PHP for some of its web applications.
Flickr: Flickr, a popular photo-sharing platform, is known for using PHP.
Tumblr: Tumblr, a microblogging and social media platform, is built using PHP.
Magento: Magento, an e-commerce platform, is written in PHP. It is widely used for building online stores.
Drupal: Drupal, a powerful and flexible content management system, is written in PHP. It is used for building a variety of websites, including government sites and corporate portals.
Joomla: Joomla is another popular open-source CMS written in PHP, used for building websites and online applications.
MailChimp: MailChimp, an email marketing platform, uses PHP in its application stack.
Please note that technology choices for websites can evolve, and some of these platforms may have made changes to their technology stacks. Additionally, new websites and applications may have emerged that also use PHP. If you know more, please don't forget to share your comments! :)