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

function my_big_function() {

    static $result;

    // Function has already run
    if ( $result !== null )
        return $result;

    // Lot of work here to determine $result
    $result="whatever";

    return $result;
}

Also see: https://stackoverflow.com/questions/6188994/static-keyword-inside-function