PHP 101

PHP, PHP Home Pages is an interpreted backend programming language. Don't fall for the somewhat derogatory term scripting language.

From your background you know that the static part of your pages may be written in plain HTML5 inside a PHP file. You also know that in dynamic pages the dynamic HTML is printed by PHP.

Example 1.1. Simplified PHP
<!DOCTYPE html>
        <meta charset="UTF-8">
        // put your code here
        $greeting = "Hello World!";
        printf("<h1>%s</h1>\n", $greeting);

See the page in your browser.