How do I create a variable on one page and use it on another?

[carlosamigos]

How to I create/set a variable on the header.php page and call it on sidebar.php?

eg.
<!-- Header.php page -->
<? // Set Random Variable
if ( is_home() ) {
$myRandomVariable = 1;
}
else {
$myRandomVariable = rand(3,5);
}
?>

<!-- Header.php page-->
<div id="headerImage_<?php echo $myRandomVariable; ?>"></div>

<!-- Sidebar.php page -->
<div id="sidebarImage_<?php echo $myRandomVariable; ?>"></div>

Thanks

Carl