PHP stands for (recursive acronym for "PHP: Hypertext Preprocessor"), a widely used general purpose scripting language release under Open Source License. It is well suited for most web development work as it can be embedded into HTML to extend the functionalities that is lacking in the HTML language. If you are interested to know more about the PHP history, here is a PHP history write up.
echo "Hello world!";
PHP is popular on Linux/UNIX web servers, as it can be bundled with apache web server which both offer a stable environment for deploying small to complex web based applications. PHP is also widely used by website developers for writing applications that interact with MySQL or Postgres database management system.
For complete reference of PHP scripting language, you can check on the Official PHP Manual.