get_header include

Hi.
Does get_header() include header.php content to the page?
similar php include('header.php')?
because, I wrote in header.php:
<?php $abcd = "1234" ?>
then wrote in index.php:

<?php
get_header();
echo $abcd;
?>

now in index page, I should see $abcd value (“1234”), but i can’t see it! and the $abcd is undefined in index.php .

Why?
and how can I solve that problem?

Thanks (sorry for bad English)