include on template files

php

Can I use PHP include function on the template files?

Reason why
example.php
hello<?php
$var = "World";
function hello_world(){
	return "world";
}
?>

header.php
<?php
require("example.php");/* or */ echo get_template_part('example');
echo $var;
echo hello_world();
?>

use require display hello Worldworld
use get_template_part('example') display hello world