If you have something expressed only in HTML that result is basically static.
Then to convert to PHP is enough to change the extension of the file (HTML) to PHP.
This first seems not to produce any advantage.
The interesting thing about PHP is that it can produce different HTML depending on many variables, like user input, system variables, session events. Also, PHP could have other instructions that have some effect on the host.
But in that case, is not “convert” the HTML but add some new behavior to it.