Why does isDot() fail on me? (PHP)

This is, because DirectoryIterator::current() (the method, that is call within a foreach-loop) returns an object, which is itself of type DirectoryIterator. FileSystemIterator (that RecursiveDirectoryIterator extends) returns an object of SplFileInfo be default. You can influence, what is return, via flags

$files = new RecursiveIteratorIterator(
  new RecursiveDirectoryIterator(
    $pathToFolder,
    FilesystemIterator::KEY_AS_PATHNAME | FilesystemIterator::CURRENT_AS_SELF));

But in your case, you don't need to test, if an item is a dot-file. Just set FilesystemIterator::SKIP_DOTS and they will not appear at all. Note, that this is also the default behavior.

The other answer is excellent, but for a different approach you can set the SKIP_DOTS flag:

<?php

$o_dir = new RecursiveDirectoryIterator($pathToFolder);
$o_dir->setFlags(RecursiveDirectoryIterator::SKIP_DOTS);
$o_iter = new RecursiveIteratorIterator($o_dir);

foreach ($o_iter as $o_info) {
   echo $o_info->getPathname(), "\n";
}

https://php.net/filesystemiterator.setflags