PHP is a widely used, general purpose, open source scripting language for writing web applications, dynamically generated pages and command line interface scripts.
Term PHP is a recursive acronym of PHP: Hypertext Preprocessor. It is not abbreviation. PHP in the beginning stood for Personal Home Pages and through progress into professional waters it was extended into the above recursive acronym.
PHP is a great language for web development. It has an enourmous community and commercial support, finding a quality and affordable hosting is very simple, and it can be used to power simple websites to professional million users per day websites.
In general, vanilla in computer software and systems means default and not customized from the original form.
Later on, you will learn how to use package management with Composer, PHP libraries, frameworks, CMSes (content management systems) and other parts of the PHP ecosystem. Vanilla PHP in this course means using PHP without any of those 3rd party libraries or frameworks or 3rd party PECL extensions. Only writing code with the built-in features that PHP provides out-of-the-box. Vanilla PHP can be also referred to as plain PHP and is here used as a convenience for understanding what this course is about.
Now let's install PHP. Proceed to the PHP installation chapter to get up and running with PHP.