I think I get what you mean, if you have PHP selected as the syntax, it will show HTML in the bottom right.
From what I've been able to find there seems to be no difference between the PHP and HTML syntax options. I have a feeling they may be 2 labels for 1 option as PHP is HTML with extras and neither in syntax really affects the other.
I'm assuming that PHP is an alias for the HTML syntax option which includes PHP syntax support, hence why it shows HTML in the bottom right. However I can't find where it affects usability?