You have asked the age old question.
Things that you ought to consider are
1) Do you really want to have lots and lots of php code in your html
2) Will designers accidentally mess it up by making inadvertant changes to the code when playing with the html page.
I can only give you my opinion but hey i expect its better than getting no opinion 🆒
I generally have the html page set up with specific areas set aside for the php code to slot into. I find that it has 1 major advantage (to having lots of php code in the page), that is it makes it easy for designers to modify / change the html design and therefore reduces the risk of breaking the code.
here is a very small example ( 2 pages index.html and page.php)
index.html
<?php
include "page.php";
?>
<html>
<head></head>
<title></title>
<body>
<?php display_message(); ?>
</body>
</html>
page.php
<?php
function display_message($message='')
{
$page_message =(!empty($message)?"$message":"hello there");
}
print $page_message;
?>
So you can see from the above example that there is only one line that the coder needs to concern themselves with.
HTH
GM