Check whether a directory exists in PHP

Should work correctly. From is_dir() documentation:

Returns TRUE if the filename exists and is a directory, FALSE otherwise.

Well, anyway if it doesn't try this:

if(file_exists($dir) && is_dir($dir))

BTW. results of these functions are cached in stat cache. Use clearstatcache() to clean that cache.

You'll probably want to use opendir() after is_dir() agrees that the path (could) be a directory.

If the resource returned by opendir() is valid, you know you have a directory, and already have a handle to read it.

Just be sure to call closedir(), either way, if a valid handle is returned.

Edit:

This answer assumes that you'll be opening the directory either way. If you just need to ensure a path is sane / valid, file_exists() is much cheaper.