Hypertext Preprocessor (or simply PHP) is a server-side scripting language designed for Web development, and also used as a general-purpose programming language.
PHP code may be embedded into HTML code, or it can be used in combination with various web template systems, content management systems, and frameworks. The web server combines the results of the interpreted and executed PHP code, which may be any type of data, including images, with the generated web page. PHP has been widely ported and can be deployed on most web servers on almost every operating system and platform, free of charge.
Learn PHP here: https://www.w3schools.com/
Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language like HTML. CSS is designed to enable the separation of presentation and content, including layout, colors, and fonts. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple web pages to share formatting by specifying the relevant CSS in a separate .css file.
Learn CSS here:
STEP 1 Learn the Basics
Responsive Web Design is used in all types of modern web development.
STEP 2 Dig Deeper
When you feel comfortable with HTML and CSS, it´s time to dig deeper.
You should learn how to use Maps, Fonts and Icons in HTML.
You should also learn how to use AJAX and JSON for making server requests.
STEP 3 Choose Frameworks
Now it is time to look at some Frameworks.
On the CSS side you should choose a framework for reponsive web design, such as Bootstrap.
React.js / Angular.js / Vue.js