guarriman on "How to handle the domain of the petitions (editing 'index.php'?)"

Working with WordPress 4.0.1., I'm trying to handle the domain of the petitions.

For that, I started to edit the file 'index.php' of my theme, and

print "My domain is: " . $_SERVER['SERVER_NAME'];
exit();

Unfortunatelly, no message is displayed. In fact, if I write wrong code, I get no error.

Which file must I edit? Thank you veyr much.

PS: I got my 'W3 Total Cache' deactivated.