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>