Hi there is there a way to modify the php.ini file from my standard php config? or should I just use .htaccess file?
About those files, is using .htaccess going to slow down the responsiveness of my website? If for example I change the extension of html to be considered as php?

I beleive the only way to change the php ini file is to install your own version of php.
A .htaccess file should not change the responsiveness of your web site.
I do not see any reason for you to change the extensions as php files are already recognized by your server.

To process .html files as PHP, you can use the .htaccess AddType directive: