Can you explain in a very simple way [if possible with example] why we need this part of PHP? Why we need OOP?

Since it's your own app, you get to decide what you want to use. The simple answer is to say "Hey, use whatever woks for you." The hard part is knowing what's going to work for you. :-)

So let's start with the first app you mentioned: Asana. I just took a look at it so it's fresh in my mind.

You're going to use HTML for the structure of the web document.

You're going to use CSS for creating the look and feel of the app.

You're going to use JavaScript to control the front-end behavior of the app. You'll probably want to use the jQuery library and the jQuery UI user interface library for the drag-and-drop functionality Asana has.

Asana doesn't reload the page when it saves data, so you're going to use AJAX to accomplish that. AJAX lets you use JavaScript to connect with a program running on your server, so you can send data to and receive data from the server without ever reloading the web page.

Since you're using JavaScript to communicate with the server, you're probably going to send and receive that data as JSON. JSON stands for JavaScript Object Notation. It's a common way to structure data that can be shared between different programs.

Now, we're finally talking about the back-end.

The most common languages you can use for back-end development are PHP, Ruby on Rails, Python, and JavaScript (via NodeJS). They're all viable back-end languages and they can all connect to the database you're going to use for data storage.

I've personally used PHP more than anything else because it's always done the trick. I'm adding JavaScript to the mix. I've written a few programs in Python (just enough to be dangerous lol), but I've never touched Ruby on Rails.

I don't think you'll go wrong learning any of the languages, but I would caution against starting with Ruby on Rails.

Ruby on Rails is a combination of a language (Ruby) and a framework (Rails). It's a great combination and lets you build web apps quickly, but it hides a lot of the complexity from the programmer.

That's great for an experienced programmer, but it prevents you from learning what's really going on under the hood. And that can bite you in the butt when something goes wrong (and it always goes wrong).

Basically, if you don't understand what Ruby on Rails is really doing, it's just going to seem like magic to you. And magic is impossible to debug because you don't understand it.

If you've got your heart set on Ruby on Rails, go for it. There's a strong community of Rails developers out there that can help you learn what's happening with it.

But if you're still not sure, try learning enough about all of them to write a simple program or two in each of them and see how they work. You'll figure out what you like and what you don't.

As far as databases go, you'll probably want to use mySQL. It's a very popular and capable open source relational database management system (RDBMS).

At it's heart, Asana is an advanced to-do list. Work up to creating a simple to-do list and saving it in a database. That's your first version.

Then add a new feature, like a log-in system so multiple users can each have their own to-do list.

Then add a new feature, like adding a project which lets store multiple to-do lists together.

You get the idea.

Have fun and ask away if you have any other questions.