Question

I want to make a web application. So I want to know can I use Ajax, JS, HTML, PHP, and MySQL together?

Answer 1

Not only you can use it together but they (or their equivalents) are all necessary to build a modern web application.

Here is a simple schematic of what each of these technologies achieve, by itself and together with the others.

1. php takes care of the server side of your application. When a user navigates to a page, the php is the first bit of code that is executed. It gets the user input, eventually queries the database for data, and outputs an html file to be interpreted by the browser;
2. html is the markup language that the browser receives when making a request to your server. It contains the data passed to it by php, put in handy containers that can be styled with css and whose content will be handled by javascript;
3. javascript. With php, you have to make a request to the server (that is, reloading the page or changing the address) every time you want data to change. Javascript, instead, is executed in the browser and allows for immediate responses. Still, with javascript alone, you won’t have access to the database, for instance. What happens if you would like to show the new messages a user received without having him reload the page? Here comes…
4. Ajax. Ajax is a way to use javascript to make requests to the server. So, in our case, javascript shall make a request to the php page that serves the new messages number – instead of the browser, by reloading. Then, it will be able to display that information right in the page the user is visiting.

If you want a comprehensive, albeit introductory, manual on these technologies and the way they interoperate, I suggest O’reilly’s “Learning PHP, MySQL and Javascript”.