How to call a function only once (global variable scope)

I have been trying to work out the most effecient way to use functions in WordPress.

I have a big slow function big_slow_function() that ideally is only run once. But I need to use the boolean that is returned by this function throughout my theme files (in header.php, page.php, sidebar.php, footer.php, loop-page.php, functions.php etc).

I am wondering how to do this.

I tried putting this in my functions.php to try and avoid calling big_slow_function() more than once:

global $my_important_boolean;

function $get_my_important_boolean()
{
    global $my_important_boolean;

    if ($my_important_boolean == NULL) // if big_slow_function() has not been run yet
        $my_important_boolean = big_slow_function();

    return $my_important_boolean;
}

And then I put code like this throughout my theme files:

if ($get_my_important_boolean()) {
    // customize content to user
}

But the big_slow_function() is still being run every time. I am not sure what I am doing wrong and have found it hard to find good documentation on variable scope in WordPress. Perhaps I need to pass a reference/pointer to the variable?

Any help with this problem is much appreciated as I have been struggling with it for sometime.