Definition: PHP is a simple, object-oriented, interpreted, robust, secure, very high-performance, architecture neutral, portable, dynamic scripting language. Everything inside "class" keyword in PHP will be identical to Java language (in near future). And PHP is designed such that it is 10 times faster than Java, since there is no virtual machine. PHP is the international standard general purpose object oriented scripting language.
PHP stands for 'Hypertext Pre-Processor' and is a server side HTML scripting/programming language. PHP is a tool that lets you create dynamic web pages. PHP-enabled web pages are treated just like regular HTML pages and you can create and edit them the same way you normally create regular HTML pages.
PHP was kept the "top secret and strictly confidential" computer language by many companies in the world, but now had become the most well-known and most widely used object oriented scripting language for web, internet, e-commerce, general purpose and business-to-business projects. Even today many competing companies keep PHP language as a highly confidential matter not disclosing to outsiders (competitors).
PHP will storm the entire world and will take the IT industry by surprise!! The power of PHP is that it is cross-platform and runs everywhere!! It runs on Linux, Windows 95/98/NT, Windows 2000, Solaris, HPUX and all flavors of unix. PHP is write once and deploy anywhere and everywhere. It runs on many web-servers like Apache, Microsoft IIS, etc..
PHP runs 5 to 20 times faster than Java!! In actual benchmarks, PHP was 3.7 times faster than JSP (see benchmarks ). PHP is extremely easy to use and you can develop very complex web/e-commerce applications very rapidly in a very short period of time. (In future PHP language will imitate most features of Java language and Java programmers will love PHP. And PHP will have java keywords like class, extends, interface, implements, public, protected, private etc..).
It has object oriented features and takes the best features from Java, C++, PERL and "C" languages. PHP language is a marriage of best features from Java, C++, PERL and C.
PHP is the real gem of all the scripting/programming languges and will soon become the "MECCA" for programmers world-wide!! PHP has a huge user base and a large developer base as it runs on both Window95/NT/2000 and all flavors of unixes.
A big surprise is waiting for us - Most probably PHP will be the computer language/scripting language of the 21st century!!
PHP can be compiled and optimized to make it run even faster by using the Zend Optimizer. Zend optimizer is integrated with PHP in PHP version 4.0.
You would normally use a combination of PHP (70% code) + HTML/DHTML/XML (25% code) + Javascript (5% code client side validations) for your e-commerce projects.