Theoretically something purely static is going to be faster than dynamically generated content. In practice the difference is very often negligible.
You have to consider that most servers cache the output of PHP scripts you can even specify for how long by file-type or per-file even. This means that in most cases, for PHP that generates largely the same output over-and-over, the server serves static content because it serves it from the cache.
Now there are times when things can be noticeably slower but I'd say those are rare for a site that can be statically generated. In other words, if you can have the same site in static HTML and generated by PHP (or anything else), you probably do not have a case where a difference is visible to your visitors. Consider that the dominant latency comes from the network and not serving the pages for anything which is not so resource intensive.