You can create an iterator for the contents of a "folder" by doing the following:
$objects = $s3->getIterator('ListObjects', array(
'Bucket' => 'bucket-name',
'Prefix' => 'subfolder-name/',
'Delimiter' => '/',
));
foreach ($objects as $object) {
// Do things with each object
}
If you just need a count, you could this:
echo iterator_count($s3->getIterator('ListObjects', array(
'Bucket' => 'bucket-name',
'Prefix' => 'subfolder-name/',
'Delimiter' => '/',
)));