both of these languages are for developing web applications but their use cases differ significantly. First of all, people who master HTML are referred to as coders, while PHP developers can proudly claim the title of a programmer.
Unofficial labels aside, HTML is one of the first subjects that every programmer-to-be is going to learn. This markup language is beginner-friendly, has tons of tutorials and courses, and is considered as the foundation of all web development. In other words, HTML is responsible for gluing the main structure of the website with its elements, attributes, and other components.
PHP is the leading back-end language for handling the server-side of websites. When compared to HTML, PHP reaches a whole different level of complexity when it comes to syntax, overall conventions of the language, and the learning curve.