PHP include: path relative to where the code is included

I have this code snippet that includes a subpage:

function getTabFile($pageName) {
    if ($pageName == null)
        return null;
    if (preg_match("/^[a-z]+$/i", $pageName) != 1)
        return null;
    $fileName = getTabFileName($pageName);
    if (file_exists($fileName))
        return $fileName;
    return null;
}
function getDefaultTabFile() {
    return getTabFileName('default');
}
function getTabFileName($page) {
    return __DIR__ . "/page/$page.php";
}
include (getTabFile($_GET['page'] ?? null) ?? getDefaultTabFile());

And I want this snippet itself to be included via:

$path = $_SERVER['DOCUMENT_ROOT']; 
$path .= "/common/snippet.php"; 
include($path);

When I do this the script targets the relative DIR of where the snippet is located.

My question is: Is it possible to target the relative DIR of the file where the snippet is included?