Actually PHP should be learnt before messing up with wordpress except for if you want to just make a blog or any other website on wordpress.

If you want to create plugins you will need PHP knowledge. I am not sure about Themes, but I was went to edit section in Wordpress dashboard for the theme that I am using and there were some .php files (They probably weren't containing only HTML) so maybe you also need PHP for themes (But I am not sure TBH)