why code don't work?

what kind of error? A syntax error, or something else? Could you show me?

 

have you tried this instead?

Code:

<?php
$int = 123;

if(filter_var($int, FILTER_VALIDATE_INT))
{
echo "Integer is valid";
}
else
{
echo "Integer is not valid";
}
?> 

and try it on the server, not your own computer (though if it is core, it shouldn't matter, but better safe than sorry)

I suspect that the ( in the echo is causing the problems. You shouldn't echo ("") like that, but like this ""

with () around the echo php thinks that being echoed is a function, which it isn't of course

! shouldn't cause problems, but try it without it anyway to see what it affects, if any

A last possible explanation is that php isn't installed on that server.