PHP is a server-side scripting language. A server-side scripting language is similar to JavaScript in many ways, as they both allow you to embed little programs (scripts) into the HTML of a Web page.

The key difference between JavaScript and PHP is simple. JavaScript is interpreted by the Web browser once the Web page that contains the script has been downloaded. Meanwhile, server-side scripting languages like PHP are interpreted by the Web server before the page is even sent to the browser.

copyright: Build Your Own Database Driven Website using PHP & MySQL by Kevin Yank