PHP (PHP: Hypertext Preprocessor) is a server-side scripting language designed for web development. It is also used as a general-purpose programming language. PHP was originally stood for Personal Home Page, however now it stands for PHP: Hypertext Preprocessor, which is a recursive backronym.
PHP code can be mixed with HTML code. PHP code is usually processed by a PHP interpreter, which is usually implemented as a web server's native module or a Common Gateway Interface (CGI) executable. After the PHP code is interpreted and executed, the web server sends resulting output to its client, usually in form of a part of the generated web page – for example, PHP code can generate a web page's HTML code, an image, or some other data. PHP has also evolved to include a command-line interface (CLI) capability and can be used in standalone graphical applications.
PHP has been widely ported and can be deployed on almost all web servers on almost every operating system and platform. And it is free of charge too.
How to upload a file using PHP script
Sample code for implementing - check availability option in registration form - using Ajax, PHP and MySQL