PHP var in wordpress

I’m very new to WP

I try to define some variables on the header for my nav bar (which is both in header and footer)

like for example:

// I declare le navigation values to be used on the 2 nav bars
$navAboutMe= get_page_link('19');

but when I want to access $navAboutMe inside the footer.php my var is empty.

why does those vars are reseted? and where? and do you have an idea on how I can get through it

thanx you in advance

Steffy