PHP is appropriate whenever you want your pages to be created dynamically when the browser requests the page. For example, you can display date, time, and other information in different ways. PHP will make your website more dynamic in content and a heck of a lot easier to update.
PHP makes it easy to customize a Web page to make it more useful for individual users.
With PHP you can set cookies, manage authentication, and redirect users.
One of PHP most powerful features is database access. With PHP it is possible to access over 19 different types of databases and manipulate data within those databases based on input from the user via a web page.
Do you have a guestbook on your web site? Do you subscribe to a third-party service that provides statistics on site visitors? Would you like to know how a user got into your web page, where they visited on your web site, and what page they left from? With PHP, such tasks are almost trivial.
PHP makes it easy to send e-mail, work with newsgroups, and actually open a connection to another web site -- and get or send data with it. If you are already familiar with ASP development, PHP can also communicate with other server-side languages such as Java and use COM interfaces.
PHP provides the means to do everything from generating PDF documents to parsing XML.
This is only a small part of possibilities that PHP provides you with. Read WebCheatSheet articles devoted to PHP to learn how to make exciting, dynamic, easy to update webpages using PHP.