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.
> 
> I don't know whether I am missing a trick in HTML. 
> 
> 
> 

Google thinks that in the future, there will be a feature where files can be included via an HTML tag.

> 
> 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. 
> 
> 
> 

Yes. Here's a code template in php that I made now that will work:

```
<?php 
$includepath="/path/to/your/includefiles";
chdir($includepath); 
?>
<!DOCTYPE HTML>
<html>
<head>
<title>Test</title>
</head>
<body>
<?php include "header.php"; ?>
<div>
<div style="float:right;">
<?php include "sidebar.php"; ?>
</div>
<!-- Main page content goes here -->
</div>
<?php include "footer.php"; ?>    
</body>
</html>

```

If you can copy that template and put all the php files in the same folder and set $includepath to that folder, then you'll be good. I used a div float box for a side bar but if you want to go old-fashioned, you could use a table as follows:

```
<!DOCTYPE HTML>
<html>
<head>
<title>Test</title>
</head>
<body>
<?php include "header.php"; ?>
<table>
<tr>
<td>
<!-- Main page content goes here -->
</td>
<td>
<?php include "sidebar.php"; ?>
</td>
</tr>
</table>
<?php include "footer.php"; ?>    
</body>
</html>

```

Only thing with a table is many people may frown on it because it isn't the best thing to use for responsive design.

> 
> Also I want to know whether this solution is good in SEO perspective or not.
> 
> 
> 

It will make no difference with SEO unless your server has a badly functioning disk with bad sectors attached to the requested files or the php engine is extremely slow. These things are extremely rare, but I'm sure they happen. Website speed affects SEO.