gistfile1.php

<?php

function chooseAmongTheseValues() {
    foreach (func_get_args() as $arg) {
        if (is_callable($arg)) {
            $value = $arg();
            if ($value) {
                return $value;
            }
        } else if ($arg) {
            return $arg;
        }
    }
}

function get($key) {
    if (array_key_exists($key, $_GET)) {
        return $_GET[$key];
    }
}

echo chooseAmongTheseValues(
    get('value'),
    function() { return 'a value i looked up'; }
);

@JamesXNelson - Strangely, this code has a limited version of eval since strings are callable if they match the name of a callable symbol. Which is, of course, a terrible idea. page.php?value=phpinfo will, for instance, get you a complete report on the php config.