I have a file "index.html". And a code into it
<?php echo "Hello world"; ?>
But in browser I can see:
<!--<?php echo "Hello world"; ?>-->
I use Microsoft-IIS/7.5server, so I cant use .htaccess file.
Save your file as index.php instead. HTML doesn't read PHP.
Edit: Try to change the home url then. Check this out:
https://www.iis.net/configreference/system.webserver/defaultdocument
You can store your file with .php extension and user rewrite module in iis to link .html file to .php file that you store. commonly for SEO Using this method.
index.php..html? It won't work in a.htmlfile because it's not HTML.