I have tried to use different PHP code inside HTML code, but everytime I do, it doesn't work, there is no output. I have also installed the PHP interpreter on Notepad++ but it didn't help.
index.php
<!DOCTYPE html>
<html>
    <head>
        <title>Hello world!</title>
    </head>
    <body>
        <?php
            echo 'Hello world!';
        ?>
    </body>
</html>
