The extension does not matter for SEO ranking, but for better experiences, we can remove the .php or .html extension from a static website.
To remove the extension we have to create a .htaccess file and put it under the website document root folder.
And then add the following code into the .htaccess file.
//put this code in the root file .htaccess
RewriteEngine on
#remove extension html
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html [NC,L]
#remove extension php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*)$ $1.php
And then remove the .php extension from your header file (if your site is still showing the .php extension).
Suppose your site is www.yourdomain.com/page.php then remove the ‘.php’ from href section.