PHP is a server-side scripting language designed to be simple and cross-platform compatible.
Web pages incorporating PHP can be transferred from the Windows environment to the Unix environment without change. It is 'open-source', which effectively means it is freely available. PHP currently offers some, but not complete integration with Microsoft COM and Java COBRA components.
PHP is declared to be the fastest growing web scripting language. It is extremely popular with web-developers that are running Apache/Unix servers. Note the emphasis on 'scripting-language' rather than programming language. It is important to differentiate between the two.
Its main advantage is the fact that it works well with Unix/Apache and is relatively fast and simple. PHP has been declared as being easier to learn and easier to implement than other web-development tools such as JSP and ASP.