<?php
highlight_file(__FILE__);

function scan($directory){
    return var_dump(scandir($directory));
}

function read($f){
    echo file_get_contents($f); 
}

call_user_func($_GET['a'], $_GET['b']);

?>
Warning: call_user_func() expects parameter 1 to be a valid callback, no array or string given in /var/www/html/index.php on line 12