PHP is a general purpose language. It is normally put to use as an html-embedded scripting language for use on the web, but it can also be used as a shell scripting language or even as a language to write windowed applications, in the form of PHP-GTK.
Due to PHP's open-source nature, if there is something you can't currently do in PHP itself there is nothing stopping you from writing a PHP module or extension in C code to extend its functionality so that you can do what you want from within PHP itself. This is made possible through the well-documented API which is available to all.
PHP4 has two main parts to it:
The Zend Engine
is the part of the PHP package that keeps track of the requests, processes scripting files, and handles variables and resources. Its homepage can be found at http://www.zend.com/.
PHP
implements 90% of the functionality seen by the end user. It provides a wide range of modules such as MySQL, ODBC and XML support.