DrTebi at yahoo dot com is wrong. is_readable() checks whether you can do file_get_contents() or similar calls, no more, no less. If the location given returns a 500 or 403 error, you can still read() that (you'll simply get the error page), but it's still read()able. Using is_readable to check the validity of a URL is simply the wrong function.

Note that is_readable() will return false for streams, eg, php://stdin.

This will return false on urls, even if file_get_contents() reads them. So, only for files.

is readable recursively. Check all sub directories and files readable
<?php
function is_readable_r($dir) {
    if (is_dir($dir)) {
        if(is_readable($dir)){
            $objects = scandir($dir);
            foreach ($objects as $object) {
                if ($object != "." && $object != "..") {
                    if (!is_readable_r($dir."/".$object)) return false;
                    else continue;
                }
            }    
            return true;    
        }else{
            return false;
        }
        
    }else if(file_exists($dir)){
        return (is_readable($dir));
        
    }
}

?>