HTML: It contains the set of all the text and media files(images, videos) that you see in a website. Kept alone, it is boring to look at or deal with.
CSS: The designer. Places the boring html set in beautiful format.
PHP: The logic maker. It decides what to present to the html to show to you. Works behind the scene. Incognito Master!
MySQL: The Store Keeper. It stores the data in an orgainsed way, ready to be presented by HTML, elegantly by CSS with the help of PHP codes.
XML: The elder brother of html. It unlike html that only displays data, can also describe the data. That is, it understands the way data is to be stored or transferred to other programs. Use this to move all your contacts from one device to another.
Now that you know how all the four mentioned above work in cooperation with one another, you must understand it’s not possible to write a complex website using only one of the four you mentioned.
Further reading: I recommend honest reading through W3Schools Online Web Tutorialsfor all of the above. They can teach you web development within 15-20 days. (You need to be sincere though.)