How do I make sure a file path is within a given subdirectory?

Question

I want to make sure a file path set via query string does not go outside of the desired subdirectory. Right now, I am checking that:

1. The path does not start w

Answer 1

The use of realpath should not change the path, so I use it in the following way:

function checkPath($pathToCheck) {
    global $basepath;
    $fullpath = $basepath.'/'.$pathToCheck;
    if ($fullpath==realpath($fullpath) && is_dir($fullpath)) {
        return $fullpath;
    } else {
        error_die('path not allowed: '.htmlentities($pathToCheck));
    }
}