Struggling to use a global variable

I am struggling to apply a global variable in a function using code snippets and wonder if there is something I am missing in the way your plugin works. My code is

$dhURL = get_the_permalink();

function facebookshare_function() {
    global $dhURL;
    $FBURL = 'https://www.facebook.com/sharer/sharer.php?u='.$dhURL;
    return $FBURL;
}
add_shortcode('facebookshare', 'facebookshare_function');

The $dhURL is not being appended when running the shortcode. It works correctly if I run

function facebookshare_function() {
    $dhURL = get_the_permalink();
    $FBURL = 'https://www.facebook.com/sharer/sharer.php?u='.$dhURL;
    return $FBURL;
}

However I wish to create other shortcodes in the same snippet (eg twitter) and use the same $dhURL variable.