I have finally figured how to get the contents of a folder (files AND "subfolders") without having to load up the entire contents of the bucket each time.
First an example array to pass in:
$options = array( "Bucket" => $bucket, "Prefix" => "rootFolder/", "Delimiter" => "/" ); This creates an Iterator that will only show you the "files":
$objects = $s3Client->getIterator('ListObjectsV2', $options); so you can only do this (which does not include "folders" because they end with "/"):
foreach ($objects as $object) { echo '<br>' . $object['Key'] . ' ' . $object['Size']; } //for
Instead, do this to get the whole object passed back:
$contents = $s3Client->listObjectsV2($options);
Then you get the whole object which contains an array called "Common Prefixes" that looks like this and gives you the "folders":
[CommonPrefixes] => Array ( [0] => Array ([Prefix] => Folder 1/) [1] => Array ([Prefix] => Carly/) )
And you get another array called Contents which is what the getIterator call returns (your files):
[Contents] => Array ( [0] => Array ( [Key] => rootFolder/myFile.txt [LastModified] => Aws\Api\DateTimeResult Object ( [date] => 2021-11-18 08:56:53.000000 [timezone_type] => 2 [timezone] => Z )
[ETag] => "d41d8cd98f00b204e9800998ecf8427e" [Size] => 0 [StorageClass] => STANDARD )
[1] => Array ...

I have finally figured how to get the contents of a folder (files AND "subfolders") without having to load up the entire contents of the bucket each time.
First an example array to pass in:
$options = array( "Bucket" => $bucket, "Prefix" => "rootFolder/", "Delimiter" => "/" );
This creates an Iterator that will only show you the "files":
$objects = $s3Client->getIterator('ListObjectsV2', $options);
so you can only do this (which does not include "folders" because they end with "/"):
foreach ($objects as $object) { echo '<br>' . $object['Key'] . ' ' . $object['Size']; } //for

Instead, do this to get the whole object passed back:
$contents = $s3Client->listObjectsV2($options);
Then you get the whole object which contains an array called "Common Prefixes" that looks like this and gives you the "folders":
[CommonPrefixes] => Array ( [0] => Array ([Prefix] => Folder 1/) [1] => Array ([Prefix] => Carly/) )
And you get another array called Contents which is what the getIterator call returns (your files):
[Contents] => Array ( [0] => Array ( [Key] => rootFolder/myFile.txt [LastModified] => Aws\Api\DateTimeResult Object ( [date] => 2021-11-18 08:56:53.000000 [timezone_type] => 2 [timezone] => Z )
[ETag] => "d41d8cd98f00b204e9800998ecf8427e" [Size] => 0 [StorageClass] => STANDARD )
[1] => Array ...