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.

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));
        
    }
}

?>

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

Do note that is_readable/is_writable evaluates permissions in an attempt to produce the result.

This WILL fail in certain situations, while the file is actually accessible to the user, but manual evaluation fails to connect the dots.

The only trusted way to detect if a file is readable is to actually open it for reading. The only trusted way to detect if a file is writable is to actually open it for writing. And catche the error in case of failure.

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