Question: I don't know whether I am missing a trick in HTML. I have one simple HTML website which has over 100 HTML pages. For each page the header, sidebar & footer section are the same but the content is different. Can I simply make this site like Wordpress where the page is formed by header.php, footer.php, page.php & sidebar.php. I know I will still have to work on page.php kind of files. 

This will simplify the file structure of the site. Also I want to know whether this solution is good in SEO perspective or not.

Answer: 
Yes, you can absolutely split your site into sections like you describe and use simple include() statements if PHP is available or SSI directives to call them. Before the rise of CMS platforms, this is how many people created sites with consistent and easy to modify sections. 

Building your pages that way has no positive or negative effect on SEO. So long as you include all of the necessary elements for SEO (title tags, meta description, rel author, etc) then your site will do as well as it did before.

What CMS's like WordPress do for you is force you into "better" (not best) practices for SEO by doing things like requiring titles for all content and automatically generating a menu system that a spider can crawl. Via the use of extensions/modules/plugins you can add more advanced SEO techniques to the site but you can also do it by hand on a non-CMS site. Ultimately working with a CMS makes things easier for some people as it automates a lot of aspects of site-building. But that same automation has drawbacks and some people prefer to hand-code everything and retain pinpoint control over all aspects of the page. Ultimately you will decide to do what is best for your development style and site needs.
Step 1:- Create a index.php in your Website folder and remove index.html.(index.html has preference over index.php)  

Step 2:- Create a header.php and footer.php and place all your code in header and footer.
Step 3:- Now include header.php and footer.php in index.php :- in place of the top header stuff, do the same for footer, head section, etc.
Step 4:- You will need to change extensions of all files to .php from .html.

```
<html> 
<head> 
<title>Page title</title> 
<?php include('header.php'); ?> 
</head> 
<body> 
enter content here
<?php include('footer.php'); ?> 
</body>
</html>

```

Another Approach :-
Step 1 :- Download Twenty Thirteen WordPress Theme and just change header.php and footer.php and sidebar.php.
Step 2 :- Publish pages and copy all html content (excluding header and footer) in the source section of every page.