If you set up a global variable in the header you would expect that it would be available everywhere, right? That is the definition of the word ‘global;, right? Well, it doesn’t work like this and the reason is because in WordPress header and footer are called via functions so the variable is only accessible in scope of each function.
One of the solutions is simple – use global variable before the declaration AND before echoing.
For example, add this to header.php:
<?php global $var; $var= "something"; ?>
and then in the footer.php you do this :
<?php global $var; echo $var; ?>
and it will echo the content of your $var variable.
Be advised though that some people say using global variables is not the best practice.