variables thru the page

greetings

I want to declare some variables that would be use within the whole site, like for example $toto=”toto”;

I tried to put them in the header.php of my theme just after the wp_head();

but then if I want to call them for example within a template page, it is like they don’t exist

is there a way to have kinda ‘global’ variable thru the whole site?

thank you in advance