how can i isolate the style (css+html) from php, like put in the php file just some lines including by it the whole style or theme.
ex:
echo
eval_template("header") .
eval_template("body") .
eval_template("footer")
So in future i can change the whole style without touch the php files any idea ?
there are many ways how you could do this...
Here's a tutorial on templating in plain PHP http://www.phpro.org/tutorials/Introduction-to-PHP-templating.html
You can also take a look at the many template engines out there. twig is one of them: http://twig.sensiolabs.org/
Personally, I enjoy to do it manually.
PHP should not return both css and html code, or even better, it shoukd not return client-side code at all but rather dependencies to specific parts. You never want to modify a file that contains a lot of different things.
To separate css+html from the php code, what I usually do is a hierarchy done with include("..."); and include_once("..."). For example : include_once("header.php") > include("menu.php") > html semantic with css classes correctly initialized according to current context.
Then you import your css / js external scripts in header.php so that you never have to modify the whole thing unless everything changes or if you have a complete feature to add to the website. Same is possible for every sections of the website.
Hope this helps, for me it is incredibly reducing debug-time since everything important has to be done only once, then at the top of it, you can seperate as you wish.
There are a lot of template engines you can use to do that, i prefer use twig, that is integrated with symfony2 framework.
Twig is wonderful because is very easy to use and very flexible, you can use inheritance to create a common layout which can be extended and overwriten in some some part using special tags. This is a guide i've find on Symfony website but is very usefull to understand the logic behind twig: http://symfony.com/doc/current/book/templating.html