PHP is a widely used web open-source web application platform, commonly used in conjunction with MySQL. Originally an abbreviation for 'Professional Home Page' the official backronym is now 'Professional Hypertext Preprocessor'. PHP is a key component of he eponymous 'LAMP' (Linux, Apache, MySQL PHP) web stack. The primary web site for php is http://www.php.net/.
If you have a question about PHP, and it's not intimately about databases, you would probably be better served on our sister site Stack Overflow