PHP and MySQL combine to be an easy yet powerful way to create dynamic web pages that actually interact with your visitors. HTML can create useful and well formatted web pages. With the addition of PHP and MySQL you can collect data from your users, create specific content on the fly, and do many other things that HTML alone can't do. PHP allows you to collect, process and utilize data to create a desired output. In short, it let's you interact with your pages.
The PHP Hypertext Preprocessor (PHP -- yes, the first "P" in the acronym does indeed stand for PHP!) allows web developers to create dynamic content that interacts with databases. PHP applications are normally found on Linux servers and in conjunction with MySQL databases. It provides those servers with functionality similar to that provided to the Windows platform by Active Server Pages technology.
The MySQL® database has become the world's most popular open source database because of its consistent fast performance, high reliability and ease of use. It's used on every continent -- Yes, even Antarctica! -- by individual Web developers as well as many of the world's largest and fastest-growing organizations to save time and money powering their high-volume Web sites, business-critical systems and packaged software -- including industry leaders such as Yahoo!, Alcatel-Lucent, Google, Nokia, YouTube, and Zappos.com. Some examples that can utilize MySQL are a web message board or a customer's shipping status.
Although PHP and MySQL can each be used independently, when you put them together it opens up countless possibilities for your site. As the internet progresses, it becomes more and more necessary to deliver dynamic content to keep up with the demands of web surfers and their desire to have information instantly delivered to them online.