Ok, it might be a banal question but i'm a little bit confused. I'm going to develop a project by my own. This means i'll build the HTML template, the PHP scripts, MYSQL query, AJAX calls and CSS styles. This means no other people will touch any part of the code.
I know templates are good ways to make the entire application easy to be modified. But is that so necessary to do it? Since i'm the only one who got to know what that files and what that page do?
Using common patterns in the development of software products is not only about making it more readable/maintainable but also about making it extensible. The longer your software will be in use, the more features will be requested/desired. Extending one big mess of a software will not work out in this case. Try a php framework (e.g. Zend Framework / Symfony(1/2)), there are lots of tutorials which allow you an easy start with or without templating engine (I use twig atm, which does a great job!).
If i were you, i wouldn't use templates (do you mean PHP frameworks?) but thats just me.
You don't have to use a template engine - PHP itself can do that just fine.
In general, it's a good idea to "separate concerns" (eg. code which outputs HTML is separate from code which loads data from the database) - it improves the readability and organization of code, making it easier to maintain and faster to develop further.
You should have a look at a framework, like symfony. Most frameworks make use of MVC and hence provide a good separation of your code.
But is that so necessary to do it? Since i'm the only one who got to know what that files and what that page do?
Yes, if you want to keep it maintainable in the long run. If you are not constantly working on the code, you will forget how some things work eventually and then you are happy if you have a proper separation and organization.
