.php extension does not affect SEO of your website however some remove it to avoid their pages looking soo ovious and some remove it to improve their websites security.

Here is how to remove the extension using .htaccess:

removing .php extension

 
                                                                                                                                                               RewriteCond %{HTTPS} off                                                                                                                     RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301] 
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]   
RewriteCond %{THE_REQUEST} /([^.]+).php [NC]
RewriteRule ^ /%1 [NC,L,R]                      
RewriteCond %{REQUEST_FILENAME}.php -f    
RewriteCond %{REQUEST_FILENAME}.php -f   
RewriteRule ^ %{REQUEST_URI}.php [NC,L]