Removing .php files for SEO

Hi,

I am brand new to Perch after having used Wordpress for many years. I've been requested by an SEO client to "fix" some "issues" that have arisen out of an SEO audit, one of which is the duplication of .html and .php filenames.

My question is with regards to the .php files and what effect it would have on the site if they were deleted. Is this recommended or should the .php filenames be redirected to the .html file instead?

Thank you.